JSON: Fix escaping of characters that have MSB=1 with signed char

The "\\u%04x" printf string did not really work in the correct way if
char is signed. Fix this by type casting this to unsigned char.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/utils/json.c

@@ -51,7 +51,7 @@ void json_escape_string(char *txt, size_t maxlen, const char *data, size_t len)
 				*txt++ = data[i];
 			} else {
 				txt += os_snprintf(txt, end - txt, "\\u%04x",
-						   data[i]);
+						   (unsigned char) data[i]);
 			}
 			break;
 		}