Assert that wrapper_handler()'s argument is within expected range.

pqsignal() already does a similar check, but strange Valgrind
reports have us wondering if wrapper_handler() is somehow getting
called with an invalid signal number.

Reported-by: Tomas Vondra <tomas@vondra.me>
Suggested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/ace01111-f9ac-4f61-b1b1-8e9379415444%40vondra.me
Backpatch-through: 17

src/port/pqsignal.c

@@ -87,6 +87,9 @@ wrapper_handler(SIGNAL_ARGS)
 {
 	int			save_errno = errno;
 
+	Assert(postgres_signal_arg > 0);
+	Assert(postgres_signal_arg < PG_NSIG);
+
 #ifndef FRONTEND
 
 	/*
@@ -123,6 +126,7 @@ pqsignal(int signo, pqsigfunc func)
 	struct sigaction act;
 #endif
 
+	Assert(signo > 0);
 	Assert(signo < PG_NSIG);
 
 	if (func != SIG_IGN && func != SIG_DFL)