gdb/python: fix gdb.Objfile.__repr__ () for dynamically compiled code

While experimenting with JIT reader API I realized that calling repr ()
on objfile created by JIT reader crashes GDB.

The problem was that objfpy_repr () called objfile_filename () which
returned NULL, causing PyString_FromFormat () to crash.

This commit fixes this problem by using objfile_name () instead of
objfile_filename (). This also makes consistent with the value of gdb.Objfile.filename variable.