compiler: check error expression in Array_type::get_backend_length

    
    Otherwise, a zero length is created in the backend and the
    backend doesn't know there is an error.
    
    Reviewed-on: https://go-review.googlesource.com/61610

From-SVN: r252849

gcc/go/gofrontend/MERGE

-4e063a8eee636cce17aea48c7183e78431174de3
+de7b370901c4fc6852eaa7372282bb699429ec4a
 
 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

@@ -7638,6 +7638,11 @@ Array_type::get_backend_length(Gogo* gogo)
   go_assert(this->length_ != NULL);
   if (this->blength_ == NULL)
     {
+      if (this->length_->is_error_expression())
+        {
+          this->blength_ = gogo->backend()->error_expression();
+          return this->blength_;
+        }
       Numeric_constant nc;
       mpz_t val;
       if (this->length_->numeric_constant_value(&nc) && nc.to_int(&val))