Check negative overflow of quickSkFDot6Div

src/core/SkAnalyticEdge.cpp

Index: src/core/SkAnalyticEdge.cpp
diff --git a/src/core/SkAnalyticEdge.cpp b/src/core/SkAnalyticEdge.cpp
index fde37e09db76014789c67cc3e9b7279847ee67f3..c17426d1db50e31668820ccebc2fc889dd112de1 100644
--- a/src/core/SkAnalyticEdge.cpp
+++ b/src/core/SkAnalyticEdge.cpp
@@ -22,8 +22,11 @@ public:
 };
 
 static inline SkFixed quickSkFDot6Div(SkFDot6 a, SkFDot6 b) {
-    if (SkAbs32(b) < kInverseTableSize) {
-        SkASSERT((int64_t)a * QuickFDot6Inverse::Lookup(b) <= SK_MaxS32);
+    // Max inverse of b is 2^6 which is 2^22 in SkFixed format.
+    // Hence the safe value of abs(a) should be less than 2^10.
+    if (SkAbs32(b) < kInverseTableSize && SkAbs32(a) < (1 << 10)) {
+        SkASSERT((int64_t)a * QuickFDot6Inverse::Lookup(b) <= SK_MaxS32
+                && (int64_t)a * QuickFDot6Inverse::Lookup(b) >= SK_MinS32);
         SkFixed ourAnswer = (a * QuickFDot6Inverse::Lookup(b)) >> 6;
         #ifdef SK_DEBUG
         SkFixed directAnswer = SkFDot6Div(a, b);