internal: fix SNPRINTF_BUFFER_SIZE macro

We need to store in 'offset' the complete amount of characters as returned from _snprintf. The value means how many characters long needs the buffer to be in order to store the corresponding string expansion.

Before this patch, in cases where the buffer is smaller than the expansion, then ret > len, and therefore ret = len.
So when incrementing offset, we do it with a wrong value.

All previous versions of libnftnl are unable to handle this situations: small buffers (or long string expansion).

BTW, if a caller must reallocate a buffer to the returned value of snprintf, it should be ret + 1.

While at it, let's add a check to know if the last snprintf call failed.

---
v2: return if ret < 0. Increment size later.

 src/internal.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index 6595e70..b06f166 100644
@@ -183,10 +183,12 @@ struct nft_set_elem {
 };
 
 #define SNPRINTF_BUFFER_SIZE(ret, size, len, offset)	\
-	size += ret;					\
+	if (ret < 0)					\
+		return ret;				\
+	offset += ret;					\
 	if (ret > len)					\
 		ret = len;				\
-	offset += ret;					\
+	size += ret;					\
 	len -= ret;
 
 #define div_round_up(n, d)	(((n) + (d) - 1) / (d))

Also applied, thanks.