osmux: Fix buffer overflow in osmux_snprintf

When running osmux_test on my PC, the process was aborted with a "stack
smashing detected" error.

Change-Id: I8a7cc422c181c0c5712ac8976a5be5f0ad44a9c0

src/osmux.c

@@ -847,7 +847,7 @@ void osmux_xfrm_output_init(struct osmux_out_handle *h, uint32_t rtp_ssrc)
 }
 
 #define SNPRINTF_BUFFER_SIZE(ret, size, len, offset)	\
-	size += ret;					\
+	size -= ret;					\
 	if (ret > len)					\
 		ret = len;				\
 	offset += ret;					\