make endmntent function handle null argument

The function originates from SunOS 4.x in which the null argument
is allowed. glibc also handles this case.

src/misc/mntent.c

@@ -10,7 +10,7 @@ FILE *setmntent(const char *name, const char *mode)
 
 int endmntent(FILE *f)
 {
-	fclose(f);
+	if (f) fclose(f);
 	return 1;
 }