man: the exists function needs to validate the first parameter

This fixes an issue with the ".so " macro for FreeBSD
ports manual pages.

PR: 275978
Reported by:	Jamie Landeg-Jones <jamie@catflap.org>
MFC after:	1 week

Differential Revision:	https://reviews.freebsd.org/D45231 (discussion)

usr.bin/man/man.sh

@@ -201,6 +201,10 @@ decho() {
 # Returns true if glob resolves to a real file and store the first
 # found filename in the variable $found
 exists() {
+	if [ -z "$1" ]; then
+		return 1
+	fi
+
 	local IFS
 
 	# Don't accidentally inherit callers IFS (breaks perl manpages)