lib/vdso: Make clock_getres() POSIX compliant again

A recent commit removed the NULL pointer check from the clock_getres()
implementation causing a test case to fault.

POSIX requires an explicit NULL pointer check for clock_getres() aside of
the validity check of the clock_id argument for obscure reasons.

Add it back for both 32bit and 64bit.

Note, this is only a partial revert of the offending commit which does not
bring back the broken fallback invocation in the the 32bit compat
implementations of clock_getres() and clock_gettime().

Fixes: a9446a90 ("lib/vdso/32: Remove inconsistent NULL pointer checks")
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1910211202260.1904@nanos.tec.linutronix.de

lib/vdso/gettimeofday.c

@@ -214,9 +214,10 @@ int __cvdso_clock_getres_common(clockid_t clock, struct __kernel_timespec *res)
 		return -1;
 	}
 
-	res->tv_sec = 0;
-	res->tv_nsec = ns;
-
+	if (likely(res)) {
+		res->tv_sec = 0;
+		res->tv_nsec = ns;
+	}
 	return 0;
 }
 
@@ -245,7 +246,7 @@ __cvdso_clock_getres_time32(clockid_t clock, struct old_timespec32 *res)
 		ret = clock_getres_fallback(clock, &ts);
 #endif
 
-	if (likely(!ret)) {
+	if (likely(!ret && res)) {
 		res->tv_sec = ts.tv_sec;
 		res->tv_nsec = ts.tv_nsec;
 	}