MFC r278297:

Fix two clang 3.6.0 warnings in usr.sbin/syslogd:

usr.sbin/syslogd/syslogd.c:1023:10: error: address of array 'f->f_prevline' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]

		    f->f_prevline && !strcmp(msg, f->f_prevline) &&
		    ~~~^~~~~~~~~~

usr.sbin/syslogd/syslogd.c:1178:16: error: address of array 'f->f_prevline' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]
} else if (f->f_prevline) {

	       ~~  ~~~^~~~~~~~~~

In both cases, the f_prevline field of struct filed is a char array, so
it can never be null. Remove the checks.

Reviewed by: jilles
Differential Revision: https://reviews.freebsd.org/D1716