curl_error: return an empty string if no error occurred

CURLOPT_ERRORBUFFER doc says "Do not rely on the contents of the
buffer unless an error code was returned." [1]

Prior to this change the error buffer was returned even if no error had
occurred, and that buffer may contain incorrect information in such a
case. [2]

[1]: https://curl.haxx.se/libcurl/c/CURLOPT_ERRORBUFFER.html
[2]: curl/curl#3629

ext/curl/interface.c

@@ -3310,8 +3310,12 @@ PHP_FUNCTION(curl_error)
 		RETURN_FALSE;
 	}
 
-	ch->err.str[CURL_ERROR_SIZE] = 0;
-	RETURN_STRING(ch->err.str);
+	if (ch->err.no) {
+		ch->err.str[CURL_ERROR_SIZE] = 0;
+		RETURN_STRING(ch->err.str);
+	} else {
+		RETURN_EMPTY_STRING();
+	}
 }
 /* }}} */