Add null pointer check in ctf_xfer_partial

I find a gdb crash when gdb reads ctf trace.  The crash is caused by passing
NULL to strcmp.  This patch is to add null pointer check, as what we did
somewhere else in ctf.c.

gdb:

2014-04-19  Yao Qi  <yao@codesourcery.com>

	* ctf.c (ctf_xfer_partial): Check 'name' is NULL before strcmp.