audit: print empty EXECVE args

Empty executable arguments were being skipped when printing out the list
of arguments in an EXECVE record, making it appear they were somehow
lost.  Include empty arguments as an itemized empty string.

Reproducer:
	autrace /bin/ls "" "/etc"
	ausearch --start recent -m execve -i | grep EXECVE
	type=EXECVE msg=audit(10/03/2018 13:04:03.208:1391) : argc=3 a0=/bin/ls a2=/etc

With fix:
	type=EXECVE msg=audit(10/03/2018 21:51:38.290:194) : argc=3 a0=/bin/ls a1= a2=/etc
	type=EXECVE msg=audit(1538617898.290:194): argc=3 a0="/bin/ls" a1="" a2="/etc"

Passes audit-testsuite.  GH issue tracker at
https://github.com/linux-audit/audit-kernel/issues/99

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[PM: cleaned up the commit metadata]
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/auditsc.c

@@ -1107,7 +1107,7 @@ static void audit_log_execve_info(struct audit_context *context,
 		}
 
 		/* write as much as we can to the audit log */
-		if (len_buf > 0) {
+		if (len_buf >= 0) {
 			/* NOTE: some magic numbers here - basically if we
 			 *       can't fit a reasonable amount of data into the
 			 *       existing audit buffer, flush it and start with