compiler: fix crashes.

The compiler would crash on:
        if true || x, y := 1, 2 {}
and
        var s string
        s = append(s, "hello")

Reported in issue 3186.

From-SVN: r185928

gcc/go/gofrontend/expressions.cc

@@ -7441,6 +7441,8 @@ Builtin_call_expression::check_one_arg()
 void
 Builtin_call_expression::do_check_types(Gogo*)
 {
+  if (this->is_error_expression())
+    return;
   switch (this->code_)
     {
     case BUILTIN_INVALID:

gcc/go/gofrontend/parse.cc

@@ -3971,7 +3971,7 @@ Parse::if_stat()
 
   bool saw_simple_stat = false;
   Expression* cond = NULL;
-  bool saw_send_stmt;
+  bool saw_send_stmt = false;
   if (this->simple_stat_may_start_here())
     {
       cond = this->simple_stat(false, &saw_send_stmt, NULL, NULL);