kill: Fix free() with bad pointer on SIG-prefixed signal-name

kill -l SIGHUP (or any other signal-name prefixed with "SIG")
would cause free() to be called with a bad pointer instead of
a pointer to what was allocated. Fix this and add test-case.
1 job for <span class="ref-name">fix-kill-l-bad-free-sig-prefix</span> in 3 minutes and 43 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #3884925
MakeCheck

00:03:43