compiler: Don't make gc symbol for bad array type.

When parsing a malformed array type, i.e. invalid length, gccgo
would loop indefinitely based off of a uninitialized length variable.

Fixes golang/go#11539.

Change-Id: Ife196c04ad7c8484dc3840360b9373ee017d41d3
Reviewed-on: https://go-review.googlesource.com/13066
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/types.cc

@@ -6423,7 +6423,10 @@ Array_type::array_gc_symbol(Gogo* gogo, Expression_list** vals,
   unsigned long bound;
   if (!this->length_->numeric_constant_value(&nc)
       || nc.to_unsigned_long(&bound) == Numeric_constant::NC_UL_NOTINT)
-    go_assert(saw_errors());
+    {
+      go_assert(saw_errors());
+      return;
+    }
 
   Btype* pbtype = gogo->backend()->pointer_type(gogo->backend()->void_type());
   int64_t pwidth = gogo->backend()->type_size(pbtype);