cpukit/posix: signal() should return SIG_ERR on error

When signum is invalid, return SIG_ERR, and set errno to
indicate the cause.

cpukit/posix/src/signal_2.c

@@ -74,6 +74,8 @@ sighandler_t signal(
   s.sa_flags   = 0;
 #endif
 
-  sigaction( signum, &s, &old );
+  if ( sigaction( signum, &s, &old ) < 0 )
+    return (SIG_ERR);
+
   return (sighandler_t) old.sa_handler;
 }

testsuites/psxtests/psxsignal01/init.c

@@ -139,6 +139,11 @@ void *POSIX_Init(
 
   TEST_BEGIN();
 
+  /* invalid signum return SIG_ERR */
+  sighandler_t old_handler =  signal( 0, SIG_DFL );
+  rtems_test_assert( old_handler == SIG_ERR );
+  rtems_test_assert( errno == EINVAL );
+
   block_all_signals();
 
   /* set the time of day, and print our buffer in multiple ways */