R2RDump - Gracefully handle inability to parse GC Info

R2RDump - Gracefully handle inability to parse GC Info

The Amd64 GC Info parser is not complete in its implementation and
sometimes will index to a location beyond the size of the image.

If we fail to parse the GC info for a method, continue dumping the rest
of the image.

Emit a warning when this happens so we don't forget about this.

Commit migrated from https://github.com/dotnet/coreclr/commit/b43650c6b60b47c5823c9bf1806fab0a5ca3f7c1