utils: fix buffer reallocation of nft_fprinft()

When _snprintf() reports it would print n characters, that n doesn't include the trailing \0 that snprintf adds.

Thus, we need to [re]allocate n+1 characters.

While at it, change the reallocation trigger. If the length of the buffer we used is equals to the expanded string length, the output has been truncated. In other words, if ret == bufsiz, then the trailing \0 is missing.

Also, check if _snprintf() returned < 0, which means an error ocurred.

---
v2: check if snprintf() calls failed. Replace calloc with malloc.

 src/utils.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

index 18917f5..20a2fa3 100644
@@ -195,17 +195,24 @@ int nft_fprintf(FILE *fp, void *obj, uint32_t type, uint32_t flags,
 	int ret;
 
 	ret = snprintf_cb(buf, bufsiz, obj, type, flags);
-	if (ret > NFT_SNPRINTF_BUFSIZ) {
-		buf = calloc(1, ret);
+	if (ret < 0)
+		goto out;
+
+	if (ret >= NFT_SNPRINTF_BUFSIZ) {
+		bufsiz = ret + 1;
+
+		buf = malloc(bufsiz);
 		if (buf == NULL)
 			return -1;
 
-		bufsiz = ret;
 		ret = snprintf_cb(buf, bufsiz, obj, type, flags);
+		if (ret < 0)
+			goto out;
 	}
 
 	ret = fprintf(fp, "%s", buf);
 
+out:
 	if (buf != _buf)
 		xfree(buf);
 

Applied, thanks Arturo.