Fixup various overflow conditions

third_party/agg23/agg_clip_liang_barsky.h

Index: third_party/agg23/agg_clip_liang_barsky.h
diff --git a/third_party/agg23/agg_clip_liang_barsky.h b/third_party/agg23/agg_clip_liang_barsky.h
index db6ca97505ad449788b9375ba21c5219c8b67a40..5b1261f004041c99538b94bcdd5237bec7850922 100644
--- a/third_party/agg23/agg_clip_liang_barsky.h
+++ b/third_party/agg23/agg_clip_liang_barsky.h
@@ -20,6 +20,7 @@
 #ifndef AGG_CLIP_LIANG_BARSKY_INCLUDED
 #define AGG_CLIP_LIANG_BARSKY_INCLUDED
 #include "agg_basics.h"
+#include "third_party/base/numerics/safe_math.h"
 namespace agg
 {
 template<class T>
@@ -36,8 +37,18 @@ inline unsigned clip_liang_barsky(T x1, T y1, T x2, T y2,
                                   T* x, T* y)
 {
     const FX_FLOAT nearzero = 1e-30f;
-    FX_FLOAT deltax = (FX_FLOAT)(x2 - x1);
-    FX_FLOAT deltay = (FX_FLOAT)(y2 - y1);
+
+    pdfium::base::CheckedNumeric<FX_FLOAT> width = x2;
+    width -= x1;
+    if (!width.IsValid())
+        return 0;
+    pdfium::base::CheckedNumeric<FX_FLOAT> height = y2;
+    height -= y1;
+    if (!height.IsValid())
+        return 0;
+
+    FX_FLOAT deltax = width.ValueOrDefault(0);
+    FX_FLOAT deltay = height.ValueOrDefault(0);
     unsigned np = 0;
     if(deltax == 0) {
         deltax = (x1 > clip_box.x1) ? -nearzero : nearzero;