bugfix(arena): Fixed crash-bug which is triggered by cloning a string into a full arena.

When allocating in a full arena, the function 'ufn_arena_alloc' returns NULL.
Up to now, the string-clone function would try to copy to there.
Now it returs NULL as string-copy when that would've happened.

inc-c/ufn/ufn_arena.h

@@ -131,6 +131,10 @@ char * ufn_arena_clone_string(ufn_arena_s *arena, const char *string)
 {
     uint32_t len_string = strlen(string);
     char *string_copy = ufn_arena_alloc(arena, len_string + 1);
+    if(string_copy == NULL)
+    {
+        return NULL;
+    }
     memcpy(string_copy, string, len_string + 1);
     return string_copy;
 }