MEDIUM: pattern: call malloc_trim() on pat_ref_reload()

This is one case where we may release large amounts of data at once. Tests
show that without this, after 10 full reloads of an ACL containing  1M IP
addresses, the memory usage grew and stabilized around 1.7 GB of RSS. With
this change, it stays around 260 MB and is stable across reloads.
diff --git a/src/pattern.c b/src/pattern.c
index 4a3fe29..e028545 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -2101,6 +2101,10 @@
 		HA_RWLOCK_WRUNLOCK(PATEXP_LOCK, &expr->lock);
 	}
 	HA_SPIN_UNLOCK(PATREF_LOCK, &ref->lock);
+
+#if defined(HA_HAVE_MALLOC_TRIM)
+	malloc_trim(0);
+#endif
 }
 
 /* This function prunes all entries of <ref>. This function