[truetype] Fix UBSan warnings on adding offsets to nullptr.

Reported as

  https://bugs.chromium.org/p/chromium/issues/detail?id=1032152

* src/truetype/ttinterp.c (Ins_FDEF, Ins_IDEF): Use `FT_OFFSET'.

ChangeLog

@@ -1,3 +1,13 @@
+2019-12-16  Werner Lemberg  <wl@gnu.org>
+
+	[truetype] Fix UBSan warnings on adding offsets to nullptr.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/chromium/issues/detail?id=1032152
+
+	* src/truetype/ttinterp.c (Ins_FDEF, Ins_IDEF): Use `FT_OFFSET'.
+
 2019-12-14  Werner Lemberg  <wl@gnu.org>
 
 	[truetype] Fix integer overflow.

src/truetype/ttinterp.c

@@ -3718,7 +3718,7 @@
     /* We will then parse the current table.                       */
 
     rec   = exc->FDefs;
-    limit = rec + exc->numFDefs;
+    limit = FT_OFFSET( rec, exc->numFDefs );
     n     = (FT_ULong)args[0];
 
     for ( ; rec < limit; rec++ )
@@ -4150,7 +4150,7 @@
     /*  First of all, look for the same function in our table */
 
     def   = exc->IDefs;
-    limit = def + exc->numIDefs;
+    limit = FT_OFFSET( def, exc->numIDefs );
 
     for ( ; def < limit; def++ )
       if ( def->opc == (FT_ULong)args[0] )