[truetype] Fix integer overflow.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19305

* src/truetype/ttinterp.c (Ins_MIRP): Use `ADD_LONG'.

ChangeLog

@@ -1,3 +1,13 @@
+2019-12-14  Werner Lemberg  <wl@gnu.org>
+
+	[truetype] Fix integer overflow.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19305
+
+	* src/truetype/ttinterp.c (Ins_MIRP): Use `ADD_LONG'.
+
 2019-12-13  Werner Lemberg  <wl@gnu.org>
 
 	Another bunch of UBSan warnings on adding offsets to nullptr.

src/truetype/ttinterp.c

@@ -6346,12 +6346,14 @@
     /* twilight points (confirmed by Greg Hitchcock)   */
     if ( exc->GS.gep1 == 0 )
     {
-      exc->zp1.org[point].x = exc->zp0.org[exc->GS.rp0].x +
-                              TT_MulFix14( cvt_dist,
-                                           exc->GS.freeVector.x );
-      exc->zp1.org[point].y = exc->zp0.org[exc->GS.rp0].y +
-                              TT_MulFix14( cvt_dist,
-                                           exc->GS.freeVector.y );
+      exc->zp1.org[point].x = ADD_LONG(
+                                exc->zp0.org[exc->GS.rp0].x,
+                                TT_MulFix14( cvt_dist,
+                                             exc->GS.freeVector.x ) );
+      exc->zp1.org[point].y = ADD_LONG(
+                                exc->zp0.org[exc->GS.rp0].y,
+                                TT_MulFix14( cvt_dist,
+                                             exc->GS.freeVector.y ) );
       exc->zp1.cur[point]   = exc->zp1.org[point];
     }