Silence ASAN int32 overflow warning

It's fine to overflow SK_MaxS32 by one, the subsequent cast ensures
correct clamping.  But we need to cast earlier in order to make ASAN
happy.

TBR=mtklein@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2013243002

Review-Url: https://codereview.chromium.org/2013243002
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 209b833..7752aac 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -612,7 +612,8 @@
         if (fx < 0) {
             // count is guaranteed to be positive, but the first arg may overflow int32 after
             // increment => casting to uint32 ensures correct clamping.
-            int n = SkTMin<uint32_t>(SkFloatToIntFloor(-fx * invDx) + 1, count);
+            int n = SkTMin<uint32_t>(static_cast<uint32_t>(SkFloatToIntFloor(-fx * invDx)) + 1,
+                                     count);
             SkASSERT(n > 0);
             fill<apply_alpha>(dstC, n, rec[0].fColor);
             count -= n;
@@ -627,7 +628,8 @@
         if (fx > 1) {
             // count is guaranteed to be positive, but the first arg may overflow int32 after
             // increment => casting to uint32 ensures correct clamping.
-            int n = SkTMin<uint32_t>(SkFloatToIntFloor((1 - fx) * invDx) + 1, count);
+            int n = SkTMin<uint32_t>(static_cast<uint32_t>(SkFloatToIntFloor((1 - fx) * invDx)) + 1,
+                                     count);
             SkASSERT(n > 0);
             fill<apply_alpha>(dstC, n, rec[fRecs.count() - 1].fColor);
             count -= n;