remote: Map invalid signal numbers to GDB_SIGNAL_UNKNOWN.

remote: Map invalid signal numbers to GDB_SIGNAL_UNKNOWN.

I realized that remote.c is not validating input here.  Currently, if
a remote stub sends in an invalid signal number (or put another way,
if a future stub sends a new signal an old GDB doesn't know about),
GDB will do out of bounds accesses in the
signal_pass/signal_stop/signal_program arrays.  It'll probably be a
long while before we add another signal number (and buggy stubs should
just be fixed), but can't hurt to be defensive.

Tested on x86_64 Fedora 17, native gdbserver.

gdb/
2013-10-22  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_parse_stop_reply) <'T'/'S'/'X' replies>: Map
	invalid signal numbers to GDB_SIGNAL_UNKNOWN.