Check snprintf for -1

src/dstr.c

@@ -279,12 +279,12 @@ int uc_dstr_vsprintf(struct DStr *str, const char *restrict fmt, va_list ap_)
     needed = vsnprintf(str->str + str->len, available, fmt, ap);
     va_end(ap);
 
-    if (needed <= 0) {
+    if (needed < 0) {
         return needed;
     }
 
     if (needed >= available) {
-        rc = uc_dstr_ensure(str, str->len + needed);
+        rc = uc_dstr_ensure(str, str->len + needed + 1);
         if (rc != 0) {
             return -1;
         }