[pset6] server.c works for cat.jpg, but not other files

My server.c program is only working for cat.jpg at the moment. I believe the issue is my load function. At the home page:

• When I click cat.jpg, the cat image shows up properly.
• When I click cat.html, favicon.ico, hello.html, and test, I get a blank white screen.
• When I click hello.php, I get a segmentation fault.

Valgrind gave me this info when I clicked on hello.php and subsequently caused the seg fault:

==31541== Invalid read of size 1
==31541==    at 0x4C31FC4: strstr (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31541==    by 0x403590: interpret (server.c:543)
==31541==    by 0x401CA1: main (server.c:255)
==31541==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==31541== 
==31541== 
==31541== Process terminating with default action of signal 11 (SIGSEGV)
==31541==  Access not within mapped region at address 0x0
==31541==    at 0x4C31FC4: strstr (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31541==    by 0x403590: interpret (server.c:543)
==31541==    by 0x401CA1: main (server.c:255)

where line 542 and 543 in server.c are as follows:

char* haystack = content;
char* needle = strstr(haystack, "\r\n\r\n");

Here is my load function:

bool load(FILE* file, BYTE** content, size_t* length)
{
    // initialize content and its length
    *content = NULL;
    *length = 0;

    // initialize buffer
    BYTE buffer[512];

    // read from file and attempt to store 512 bytes in buffer
    while (fread(buffer, 512, 1, file) != 0)
    {
        // append bytes to content 
        *content = realloc(*content, *length + 512 + 1);
        if (*content == NULL)
        {
            printf("Memory allocation error.\n");
            *length = 0;
            return false;
        }
        memcpy(*content + *length, buffer, 512);
        *length += 512;

        // null-terminate message thus far
        *(*content + *length) = '\0';
    }

    // if EOF reached
    if (feof(file))
    {
        return true;
    }
    // if error occurred
    else
    {
        if (*content != NULL)
        {
            free(*content);
            *content = NULL;
        }
        *length = 0;
        return false;
    }
}

Can someone point me in the right direction? I've been stuck on this for a while and would appreciate some guidance. Thanks!