compiler: detect failure to set init priority for package

    
    This would have caught the problem fixed by https://golang.org/cl/43610.
    
    Reviewed-on: https://go-review.googlesource.com/43999

From-SVN: r248394

gcc/go/gofrontend/MERGE

-369e1efe19adfc5393d2235992327f39360e0554
+ec49c69df1df4d62f3751fcd7e930d6508d67bf2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -689,7 +689,13 @@ Gogo::init_imports(std::vector<Bstatement*>& init_stmts, Bfunction *bfunction)
 	 this->imported_init_fns_.begin();
        p != this->imported_init_fns_.end();
        ++p)
-    v.push_back(*p);
+    {
+      if ((*p)->priority() < 0)
+	go_error_at(Linemap::unknown_location(),
+		    "internal error: failed to set init priority for %s",
+		    (*p)->package_name().c_str());
+      v.push_back(*p);
+    }
   std::sort(v.begin(), v.end(), priority_compare);
 
   // We build calls to the init functions, which take no arguments.