Fix a crash in exif processing

Given a zero length string, the pointer was left unset, but
later code checked if the value of the pointer was non-null,
and ignored the length.

hphp/runtime/ext/ext_image.cpp

@@ -6016,6 +6016,7 @@ static int exif_process_string_raw(char **result, char *value,
    * chars up to byte_count, we also have to add a single NUL character to
    * force end of string.
    */
+  *result = 0;
   if (byte_count) {
     (*result) = (char*)IM_MALLOC(byte_count + 1);
     CHECK_ALLOC_R((*result), byte_count + 1, 0);