tccgen: gen_assign_cast(): cannot cast struct to scalar

The case below previously was causing an assertion failure
in the target specific generator.

It probably is not incorrect not to allow this even if
gcc does.

    struct S { long b; };

    void f(struct S *x)
    {
        struct S y[1] = { *x };
    }

tccgen.c

@@ -2523,10 +2523,13 @@ static void gen_assign_cast(CType *dt)
     case VT_LLONG:
         if (sbt == VT_PTR || sbt == VT_FUNC) {
             tcc_warning("assignment makes integer from pointer without a cast");
+        } else if (sbt == VT_STRUCT) {
+            goto case_VT_STRUCT;
         }
         /* XXX: more tests */
         break;
     case VT_STRUCT:
+    case_VT_STRUCT:
         tmp_type1 = *dt;
         tmp_type2 = *st;
         tmp_type1.t &= ~(VT_CONSTANT | VT_VOLATILE);