Cygwin: /proc/<PID>/status: avoid crash computing signal info

The code computing the mask of pending signals used the per-queued
signal TLS pointer without checking it for NULL.  Fix this by using
the process-wide signal mask in that case.

Fixes: 195169186b ("Cygwin: wait_sig: allow to compute process-wide mask of pending signals")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.4.4

@@ -10,3 +10,6 @@ Bug Fixes
 
 - Reinstantiate exporting _alloca.
   Addresses: https://cygwin.com/pipermail/cygwin/2023-January/252797.html
+
+- Avoid hangs when reading /proc/<PID>/status.
+  Addresses: https://cygwin.com/pipermail/cygwin/2022-December/252756.html

winsup/cygwin/sigproc.cc

@@ -1375,7 +1375,8 @@ wait_sig (VOID *)
 	    *pack.mask = 0;
 	    while ((q = q->next))
 	      {
-		if (q->sigtls->sigmask & (bit = SIGTOMASK (q->si.si_signo)))
+		_cygtls *sigtls = q->sigtls ?: _main_tls;
+		if (sigtls->sigmask & (bit = SIGTOMASK (q->si.si_signo)))
 		  {
 		    tl_entry = cygheap->find_tls (q->si.si_signo, issig_wait);
 		    if (tl_entry)