compiler: stop iterator on error

If there is an error reading or parsing an archive header, the
    Archive_iterator code would return a dummy header but would not mark
    itself as done.  The effect is that an invalid archive leads to an
    endless loop reading and re-reading the same archive header.  Avoid
    that by setting the offset to the end of the file, which will cause
    the iterator to == archive_end.
    
    No test since it doesn't seem worth constructing an invalid archive.
    
    Reviewed-on: https://go-review.googlesource.com/c/158217

From-SVN: r268129

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c76ba3014e42cc6adc3d43709bba28c5ad7a6ba2
+e7427654f3af83e1feea727a62a97172d7721403
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/import-archive.cc

@@ -780,6 +780,7 @@ Archive_iterator::read_next_header()
                                      &this->next_off_))
 	{
 	  this->header_.off = filesize;
+	  this->off_ = filesize;
 	  return;
 	}
       this->header_.off = this->off_;