Go patch committed: Don't crash on erroneous channel reads

[Ian Lance Taylor]

This patch by Chris Manghane fixes the Go frontend to not crash on
erroneous channel reads.  This fixes https://golang.org/issue/12320 .
Bootstrapped and ran Go testsuite on x86_64-unknown-linux-gnu.
Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 227830)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-79f457a267ff2bf8e65db4bcec0cbc7add79227a
+1cb26dc898bda1e85f4dd2ee204adbce792e4813
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/expressions.cc
===================================================================
--- gcc/go/gofrontend/expressions.cc    (revision 227830)
+++ gcc/go/gofrontend/expressions.cc    (working copy)
@@ -3472,6 +3472,15 @@ Unsafe_type_conversion_expression::do_ge
 
   Type* t = this->type_;
   Type* et = this->expr_->type();
+
+  if (t->is_error_type()
+      || this->expr_->is_error_expression()
+      || et->is_error_type())
+    {
+      go_assert(saw_errors());
+      return context->backend()->error_expression();
+    }
+
   if (t->array_type() != NULL)
     go_assert(et->array_type() != NULL
               && t->is_slice_type() == et->is_slice_type());