Does returning a pointer from a function make it invalid to reallocate?

So I'm currently trying to write a C program to track the longest word(s) from argv. It's been going great! Until I tried to reallocate a character double pointer, it seems to think it's an invalid pointer.

The exact error I'm getting is;

realloc(): invalid pointer
fish: Job 1, './longest-strings.o hello...' terminated by signal SIGABRT (Abort)

I'm creating this double character pointer through the return of a function, is this possibly the error? I'm pretty sure my use of realloc is correct, and I can't quite seem to trace the issue.

Any help would be massively appreciated!

/* 
 * Author: Smallblue2
 * Description: Program finds the longest word in an input string
 *
 * Input: A string from cmd line
 * Output: The longest word in a string
 */

// Header files
#include <stdio.h>
#include <stdlib.h>

// Function prototypes
int stringLength(char *string);
void longestWords(char **strings, int amt);
char **reset(char *string);
void display(char **longest, int len_array);

// Main function
int main(int argc, char **argv)
{
    char **strings = &*(argv + 1);
    longestWords(strings, argc - 1);
    return 0;
}

// Find the length of a string
int stringLength(char *string)
{
    int length = 0;
    while (*string != '\0')
    {
        length++;
        string++;
    }
    return length;
}

// Finds the longest word(s) from argv
void longestWords(char **strings, int amt)
{
    // Set up variables & pointers
    int len_array = 1;
    // Assign the first string to be the longest
    char **longest = reset(*(strings));
    int longest_len = stringLength(*(longest));
    int length = 0;
    // Loop through the rest of the strings
    for (int i = 1; i < amt; i++)
    {
        // Find the length of the current string
        length = stringLength(*(strings + i));
        // If it is larger, reset the longest array and place the
        // new string inside
        if (length > longest_len)
        {
            longest_len = length;
            longest = reset(*(strings + i));
            len_array = 1;
        // Else, expand the longest array's memory and add the
        // additional string inside
        } else if (length == longest_len) {
            len_array++;
            char **temp_longest = (char **)realloc(longest, len_array * sizeof(char *));
            if (!longest)
            {
                printf("Error: Memory allocation failed!\n");
                free(longest);
                return;
            }
            longest = temp_longest;
            *(longest + len_array - 1) = *(strings + i);
        }
    }
    // Display the longest word(s)
    display(longest, len_array);
    free(longest);
    longest = NULL;
    return;
}

// Resets the longest word array
char **reset(char *string)
{
    char **longest = (char **)malloc(sizeof(char *));
    if (!longest)
    {
        printf("Error: Memory Allocation Failed!\n");
        return NULL;
    }
    longest = &string;
    return longest;
}

// Displays the longest word(s)
void display(char **longest, int len_array)
{
    for (int i = 0; i < len_array; i++)
    {
        printf("%s\n", *(longest + i));
    }
    return;
}

I've tried to use both calloc and malloc, I tried executing the script where realloc wouldn't occur and then apparently free() believes there's an invalid pointer too. Really lost here.

Here are the two minimal changes:

1. stringLength should handle a NULL pointer.

int stringLength(char *string)
{
    int length = 0;
    while (string && *string != '\0')
    {
        length++;
        string++;
    }
    return length;
}

Or perhaps:

#include <string.h>

size_t stringLength(char *string)
{
    return string ? strlen(string) : 0;
}

2. reset() leaks the memory you just allocated, and you don't want to take the address of an argument which is out of scope when the function returns. Not entirely sure what the point of the function is but try this instead:

char **reset(char *string)
{
    char **longest = malloc(sizeof(char *));
    if (!longest)
    {
        printf("Error: Memory Allocation Failed!\n");
        return NULL;
    }
    *longest = string;
    return longest;
}

and example output:

$ ./a.out hello...
hello...

./a.out hello world!
world!