Fix pointer comparision with undefined behavior.

This didn't cause bad code generation in the one case spot-checked (gcc
4.8.1), but had the potential to to so.  This bug was introduced by
594c759f37c301d0245dc2accf4d4aaf9d202819 (Optimize
arena_prof_tctx_set().).
diff --git a/include/jemalloc/internal/arena.h b/include/jemalloc/internal/arena.h
index 76c5b93..4c1a471 100644
--- a/include/jemalloc/internal/arena.h
+++ b/include/jemalloc/internal/arena.h
@@ -1105,8 +1105,8 @@
 
 		assert(arena_mapbits_allocated_get(chunk, pageind) != 0);
 
-		if (unlikely(usize > SMALL_MAXCLASS || tctx >
-		    (prof_tctx_t *)(uintptr_t)1U)) {
+		if (unlikely(usize > SMALL_MAXCLASS || (uintptr_t)tctx >
+		    (uintptr_t)1U)) {
 			arena_chunk_map_misc_t *elm;
 
 			assert(arena_mapbits_large_get(chunk, pageind) != 0);