Fix stack-use-after-scope in typemap

The value `a` is referenced in a GC frame past its liftetime. In
practice this probably causes few issues, but in theory, the
compiler is allowed to start scribbling garbage into the stack
slot as soon as it goes out of scope. Asan complains about this
(correctly).

src/typemap.c

@@ -349,14 +349,13 @@ int jl_typemap_visitor(jl_typemap_t *cache, jl_typemap_visitor_fptr fptr, void *
             goto exit;
         JL_GC_POP();
         return 1;
+exit:
+        JL_GC_POP();
+        return 0;
     }
     else {
         return jl_typemap_node_visitor((jl_typemap_entry_t*)cache, fptr, closure);
     }
-
-exit:
-    JL_GC_POP();
-    return 0;
 }
 
 static unsigned jl_supertype_height(jl_datatype_t *dt)