compiler: avoid crash on erroneous type

If there is an error constructing the backend type, the GCC backend
    will report that the size is 1.  That will then cause construction of
    the ptrmask to crash.  Avoid that case by just generating an empty
    ptrmask.
    
    Noticed while compiling a broken package.  The policy I've been
    following is to not commit a test case for a compiler crash on invalid
    code, so no test case.
    
    Reviewed-on: https://go-review.googlesource.com/45775

From-SVN: r249208

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-372e75503c1dc9a38d9978aa6b67631283d5d6dd
+6449e2832eef94eacf89c88fa16bede637f729ba
 
 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

@@ -2570,16 +2570,16 @@ Type::make_gc_symbol_var(Gogo* gogo)
 bool
 Type::needs_gcprog(Gogo* gogo, int64_t* ptrsize, int64_t* ptrdata)
 {
+  Type* voidptr = Type::make_pointer_type(Type::make_void_type());
+  if (!voidptr->backend_type_size(gogo, ptrsize))
+    go_unreachable();
+
   if (!this->backend_type_ptrdata(gogo, ptrdata))
     {
       go_assert(saw_errors());
       return false;
     }
 
-  Type* voidptr = Type::make_pointer_type(Type::make_void_type());
-  if (!voidptr->backend_type_size(gogo, ptrsize))
-    go_unreachable();
-
   return *ptrdata / *ptrsize > max_ptrmask_bytes;
 }
 
@@ -2795,7 +2795,13 @@ Bvariable*
 Type::gc_ptrmask_var(Gogo* gogo, int64_t ptrsize, int64_t ptrdata)
 {
   Ptrmask ptrmask(ptrdata / ptrsize);
+  if (ptrdata >= ptrsize)
     ptrmask.set_from(gogo, this, ptrsize, 0);
+  else
+    {
+      // This can happen in error cases.  Just build an empty gcbits.
+      go_assert(saw_errors());
+    }
   std::string sym_name = "runtime.gcbits." + ptrmask.symname();
   Bvariable* bvnull = NULL;
   std::pair<GC_gcbits_vars::iterator, bool> ins =