Fix PR gdb/21364: Dead code due to an unreachable condition in osdata.c

Fix PR gdb/21364: Dead code due to an unreachable condition in osdata.c

Pedro's recent commits enabling -Wwrite-strings has changed a bit the
logic of info_osdata.  Now, 'type' is always non-NULL, so we have to
check if it's an empty string instead of NULL.  One of the checks was
fixed, but there is another that was left behind.  This commit fixes
it.

gdb/ChangeLog:
2017-04-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21364
	* osdata.c (info_osdata): Check if 'type' is an empty string
	instead of NULL.