Fix error handling in retry_snprintf function

Signed-off-by: vast0906 <vast0906@gmail.com>

Author: vast0906

src/flb_sds.c

@@ -403,14 +403,19 @@ int flb_sds_snprintf(flb_sds_t *str, size_t size, const char *fmt, ...)
  retry_snprintf:
     va_start(va, fmt);
     ret = vsnprintf(*str, size, fmt, va);
-    if (ret >= size) {
+    if (ret < 0) {
+        va_end(va);
+        return -1;
+    }
+    if ((size_t) ret >= size) {
         /* +1 for NUL terminator; vsnprintf writes at most size-1 chars. */
-        tmp = flb_sds_increase(*str, ret - size + 1);
+        tmp = flb_sds_increase(*str, (size_t) ret - size + 1);
         if (tmp == NULL) {
+            va_end(va);
             return -1;
         }
         *str = tmp;
-        size = ret + 1;
+        size = (size_t) ret + 1;
         va_end(va);
         goto retry_snprintf;
     }