[psaux] Fix integer overflow.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30154

* src/psaux/psblues.c (cf2_blues_capture): Use `SUB_INT32`.

ChangeLog

@@ -1,3 +1,13 @@
+2021-02-03  Werner Lemberg  <wl@gnu.org>
+
+	[psaux] Fix integer overflow.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30154
+
+	* src/psaux/psblues.c (cf2_blues_capture): Use `SUB_INT32`.
+
 2021-02-02  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	* builds/unix/configure.raw [mmap support]: Explicitly handle Windows.

src/psaux/psblues.c

@@ -506,7 +506,8 @@
             /* guarantee minimum of 1 pixel overshoot */
             dsNew = FT_MIN(
                       cf2_fixedRound( bottomHintEdge->dsCoord ),
-                      blues->zone[i].dsFlatEdge - cf2_intToFixed( 1 ) );
+                      SUB_INT32 ( blues->zone[i].dsFlatEdge,
+                                  cf2_intToFixed( 1 ) ) );
           }
 
           else