[hb-shape-fallback] Use signed int for correction

src/hb-ot-shape-fallback.cc

diff --git a/src/hb-ot-shape-fallback.cc b/src/hb-ot-shape-fallback.cc
index 42bf524..7d00a35 100644
--- a/src/hb-ot-shape-fallback.cc
+++ b/src/hb-ot-shape-fallback.cc
@@ -301,7 +301,7 @@
       /* Don't shift down "above" marks too much. */
       if ((y_gap > 0) != (pos.y_offset > 0))
       {
-	unsigned int correction = -pos.y_offset / 2;
+	int correction = -pos.y_offset / 2;
 	base_extents.y_bearing += correction;
 	base_extents.height -= correction;
 	pos.y_offset += correction;