protect against printf errors (#319)

cutils.c

@@ -176,6 +176,8 @@ int __attribute__((format(printf, 2, 3))) dbuf_printf(DynBuf *s,
     va_start(ap, fmt);
     len = vsnprintf(buf, sizeof(buf), fmt, ap);
     va_end(ap);
+    if (len < 0)
+        return -1;
     if (len < sizeof(buf)) {
         /* fast case */
         return dbuf_put(s, (uint8_t *)buf, len);