sys-process/cronbase: second attempt at fixing bug 595492

In the case of a stale lockfile, the old code still produces error
messages:

/usr/sbin/run-crons: line 61: /proc/6450/cmdline: No such file or
directory

This change should dodge that scenario.

sys-process/cronbase/files/run-crons-0.3.7

@@ -58,7 +58,7 @@ grab_lock() {
 		# another run-crons process.
 		# The tr call deletes null bytes so newer bash versions do not complain
 		# about them.
-		cmdline1=$(tr -d '\0'  < "/proc/${cronpid}/cmdline" 2>/dev/null) || :
+		cmdline1=$([ -r "/proc/${cronpid}/cmdline" ] && tr -d '\0'  < "/proc/${cronpid}/cmdline" 2>/dev/null) || :
 		cmdline2=$(tr -d '\0'  < /proc/$$/cmdline)
 		if [ "${cmdline1}" = "${cmdline2}" ] ; then
 			# Whoa, another run-crons is really running.