Initialize variable and silence GCC warning from last commit

Initialize variable and silence GCC warning from last commit

BuildBot e-mailed me to let me know that my last commit broke GDB on
RHEL-7.1 s390x.  On solib-svr4.c:svr4_handle_solib_event, 'val' now
needs to be initialized as NULL because it is inside a TRY..CATCH
block.  This patch does that.  Pushed as obvious.

gdb/ChangeLog:
2015-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* solib-svr4.c (svr4_handle_solib_event): Initialize 'val' as NULL