Fix mem leak when merging incompatible types.

Though this would typically be caused by a scripting error that should
be fixed and Bro would exit anyway if encountered at parse time.

src/Type.cc

@@ -2003,6 +2003,7 @@ BroType* merge_types(const BroType* t1, const BroType* t2)
 			if ( ! y1 || ! y2 )
 				{
 				t1->Error("incompatible types", t2);
+				Unref(tl3);
 				return 0;
 				}