xrealloc modification

src_libmhe_xmalloc.c

   void* new_ptr = realloc(ptr, size);
-  if (!new_ptr) {
-    // If size is zero, free() was called, instead of realloc(). Don't use after free().
-    if (size != 0) {
-      fprintf(stderr, "\nxrealloc(%p, %zu): %s\n", ptr, size, strerror(errno));
-      free(ptr);
-    }
+  // If size is zero, realloc() is equivalent to free().
+  if (!new_ptr && size != 0) {
+    fprintf(stderr, "\nxrealloc(%p, %zu): %s\n", ptr, size, strerror(errno));
+    free(ptr);
     exit(errno);
   }
   return new_ptr;