compiler: Don't crash on erroneous array types.

Fixes golang/go#11546.
    
    Reviewed-on: https://go-review.googlesource.com/13795

From-SVN: r227184

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-81810917af7ba19e1f9f8efc8b1989f7d6419d30
+d6d59d5927c4ea0c02468ebc6a2df431fb64595a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -5781,6 +5781,8 @@ Array_type::verify_length()
 bool
 Array_type::do_verify()
 {
+  if (this->element_type()->is_error_type())
+    return false;
   if (!this->verify_length())
     this->length_ = Expression::make_error(this->length_->location());
   return true;