gdb/linux-nat: don't memset siginfo on failure in linux_nat_get_siginfo

No caller cares about the value of *SIGINFO on failure.  It's also
documented in the function doc that *SIGINFO is uninitialized (I
understand "untouched") on failure.

Change-Id: I5ef38a5f58e3635e109b919ddf6f827f38f1225a
1 file changed