infiniband-diags: Fix memory leak in dump_multicast_tables

Coverity detected leak.

Submitted by:	bret_ketchum@dell.com
Reported by:	Coverity
Reviewed by:	hselasky, kib
MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D26228

contrib/ofed/infiniband-diags/src/ibroute.c

@@ -222,6 +222,7 @@ char *dump_multicast_tables(ib_portid_t * portid, unsigned startlid,
 				fprintf(stderr, "SubnGet() failed"
 						"; MAD status 0x%x AM 0x%x\n",
 						status, mod);
+				free(mapnd);
 				return NULL;
 			}
 		}