pal_signal: add missing mutex unlock when SIGCHLD==SIG_IGN

[tmds]

Child reapping was changed to be triggered by the SIGCHLD signal (#26291).
As part of that change, code was added to handle the original handler being SIG_IGN.
In that case, there was a missing mutex unlock.

Fixes https://github.com/dotnet/corefx/issues/29841.

cc: @wfurt @janvorli @mateusrodrigues

[tmds]

Some background:
When SIGCHLD is set to SIG_DFL, then the signal doesn't do anything.
When a user sets SIGCHLD to SIG_IGN, then the kernel will clean up the children without requiring the application to waitpid. So when we override that signal, we are responsible for cleaning up those zombies.
It is not likely a user will do this, it was part of #26291 mostly for completeness.

[stephentoub]

It is not likely a user will do this

Isn't SIG_IGN the default disposition for SIGCHLD?

[tmds]

Isn't SIG_IGN the default disposition for SIGCHLD?

The naming is confusing. The default disposition is to ignore SIGCHLD (SIG_DFL), and this is different from when the disposition is set to SIG_IGN.

From the man page: http://man7.org/linux/man-pages/man2/sigaction.2.html

POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN.
POSIX.1-2001 and later allow this possibility, so that ignoring
SIGCHLD can be used to prevent the creation of zombies (see wait(2)).
Nevertheless, the historical BSD and System V behaviors for ignoring
SIGCHLD differ, so that the only completely portable method of
ensuring that terminated children do not become zombies is to catch
the SIGCHLD signal and perform a wait(2) or similar.