Fix assertion in malloc.c:tcache_get.

One of the warnings that appears with -Wextra is "ordered comparison
of pointer with integer zero" in malloc.c:tcache_get, for the
assertion:

  assert (tcache->entries[tc_idx] > 0);

Indeed, a "> 0" comparison does not make sense for
tcache->entries[tc_idx], which is a pointer.  My guess is that
tcache->counts[tc_idx] is what's intended here, and this patch changes
the assertion accordingly.

Tested for x86_64.

	* malloc/malloc.c (tcache_get): Compare tcache->counts[tc_idx]
	with 0, not tcache->entries[tc_idx].

(cherry picked from commit 77dc0d8643aa99c92bf671352b0a8adde705896f)

ChangeLog

@@ -1,3 +1,8 @@
+2019-02-04  Joseph Myers  <joseph@codesourcery.com>
+
+	* malloc/malloc.c (tcache_get): Compare tcache->counts[tc_idx]
+	with 0, not tcache->entries[tc_idx].
+
 2019-02-06  Stefan Liebler  <stli@linux.ibm.com>
 
 	[BZ #23403]

malloc/malloc.c

@@ -2948,7 +2948,7 @@ tcache_get (size_t tc_idx)
 {
   tcache_entry *e = tcache->entries[tc_idx];
   assert (tc_idx < TCACHE_MAX_BINS);
-  assert (tcache->entries[tc_idx] > 0);
+  assert (tcache->counts[tc_idx] > 0);
   tcache->entries[tc_idx] = e->next;
   --(tcache->counts[tc_idx]);
   e->key = NULL;