use SkRect::outset, as it clamps on overflow

Bug: oss-fuzz:7929
Change-Id: Ic9a2582eb7ab638fda71b10bf987a6c584770e43
Reviewed-on: https://skia-review.googlesource.com/122959
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index fa2c08a..487aa41 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1578,12 +1578,10 @@
     }
 
     SkRect bounds;
-    SkRect r;
     // adjust it outwards in case we are antialiasing
-    const int inset = 1;
+    const int margin = 1;
 
-    r.iset(ibounds.fLeft - inset, ibounds.fTop - inset,
-           ibounds.fRight + inset, ibounds.fBottom + inset);
+    SkRect r = SkRect::Make(ibounds.makeOutset(margin, margin));
     inverse.mapRect(&bounds, r);
     return bounds;
 }