allows mallocmapping copies of size zero (#35803)

Author: gaaclarke

fml/mapping.h

@@ -160,7 +160,7 @@ class MallocMapping final : public Mapping {
   /// for `uint8_t` and `char`.
   template <typename T>
   static MallocMapping Copy(const T* begin, const T* end) {
-    FML_DCHECK(end > begin);
+    FML_DCHECK(end >= begin);
     size_t length = end - begin;
     return Copy(begin, length);
   }

fml/mapping_unittests.cc

@@ -60,4 +60,10 @@ TEST(MallocMapping, IsDontNeedSafe) {
   ASSERT_FALSE(mapping.IsDontNeedSafe());
 }
 
+TEST(MallocMapping, CopySizeZero) {
+  char ch = 'a';
+  MallocMapping mapping = MallocMapping::Copy(&ch, &ch);
+  ASSERT_EQ(0u, mapping.GetSize());
+}
+
 }  // namespace fml