Initialize yet another variable to silence GCC warning from last-but-one commit

Initialize yet another variable to silence GCC warning from last-but-one commit

Yet another BuildBot e-mail, yet another breakage on RHEL-7.1 s390x
(which uses an older GCC).  This time,
solib-svr4.c:solib_event_probe_action has the probe_argc variable,
which is now inside a TRY..CATCH and therefore needs to be
initialized.  Pushed as obvious.

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

	* solib-svr4.c (solib_event_probe_action): Initialize 'probe_argc'
	as zero.