[core] perf: small improvement buffer_string_space

src/buffer.h

@@ -201,7 +201,7 @@ static inline size_t buffer_string_length(const buffer *b) {
 }
 
 static inline size_t buffer_string_space(const buffer *b) {
-	return NULL != b && b->size ? b->size - b->used - (0 == b->used) : 0;
+	return NULL != b && b->size ? b->size - (b->used | (0 == b->used)) : 0;
 }
 
 static inline void buffer_append_string_buffer(buffer *b, const buffer *src) {