Check for null map in matchtag().

sourcepawn/compiler/tests/fail-assign-null-to-any.sp

@@ -0,0 +1,4 @@
+Foo(any foo) {}
+public Bar() {
+  Foo(null);
+}

sourcepawn/compiler/tests/fail-assign-null-to-any.txt

@@ -0,0 +1 @@
+(3) : error 148: cannot assign null to a non-nullable type