core: Fix off-by-one assertion in mctp_pktbuf_alloc_end

pkt->end refers to the address after the last byte allocated to the
packet body, so we need a less-than-or-equal-to inequality.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ie91f4fe015858b5d2f308289e806d85d8320f239
diff --git a/core.c b/core.c
index 8a1796f..6131a3c 100644
--- a/core.c
+++ b/core.c
@@ -121,7 +121,7 @@
 {
 	void *buf;
 
-	assert(size < (pkt->size - pkt->end));
+	assert(size <= (pkt->size - pkt->end));
 	buf = pkt->data + pkt->end;
 	pkt->end += size;
 	return buf;