Nuking the temporary pointer once it is properly tracked in local storage.

This should fix the double free() bug where there's no tailing newline(\n)
character:

	current# echo -n test | tail
	testAssertion failed: (run->magic == ARENA_RUN_MAGIC), function
	arena_dalloc, file /usr/src/lib/libc/stdlib/malloc.c, line 2448.
	Abort (core dumped)

Reviewed by:	kib
MFC after:	3 days

usr.bin/tail/read.c

@@ -184,6 +184,7 @@ lines(FILE *fp, off_t off)
 	}
 	if (cnt) {
 		llines[recno].l = sp;
+		sp = NULL;
 		llines[recno].len = cnt;
 		if (++recno == off) {
 			wrap = 1;