Fix unused variable warning. am: ae97b9296b am: d9dbec9512

Original change: https://android-review.googlesource.com/c/platform/external/dlmalloc/+/3021330

Change-Id: Ib7d2422274a350d6d95929a155125373f006167a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

dlmalloc.c

diff --git a/dlmalloc.c b/dlmalloc.c
index a61c0da..3eb761d 100644
--- a/dlmalloc.c
+++ b/dlmalloc.c
@@ -4009,7 +4009,8 @@
   msegmentptr ss = (msegmentptr)(chunk2mem(sp));
   mchunkptr tnext = chunk_plus_offset(sp, ssize);
   mchunkptr p = tnext;
-  int nfences = 0;
+  /* Only used in assert. */
+  [[maybe_unused]] int nfences = 0;
 
   /* reset top to new space */
   init_top(m, (mchunkptr)tbase, tsize - TOP_FOOT_SIZE);