Add bounds check in get_ptr.

Summary: `get_ptr()` checks if the index is past the end of the array, but it
doesn't check if it is less than 0, while `at()` does.

Reviewed By: @yfeldblum

Differential Revision: D2258548

folly/dynamic.cpp

@@ -165,7 +165,7 @@ const dynamic* dynamic::get_ptr(dynamic const& idx) const {
     if (!idx.isInt()) {
       throw TypeError("int64", idx.type());
     }
-    if (idx >= parray->size()) {
+    if (idx < 0 || idx >= parray->size()) {
       return nullptr;
     }
     return &(*parray)[idx.asInt()];

folly/test/DynamicTest.cpp

@@ -269,6 +269,7 @@ TEST(Dynamic, ObjectForwarding) {
 TEST(Dynamic, GetPtr) {
   dynamic array = { 1, 2, "three" };
   EXPECT_TRUE(array.get_ptr(0));
+  EXPECT_FALSE(array.get_ptr(-1));
   EXPECT_FALSE(array.get_ptr(3));
   EXPECT_EQ(dynamic("three"), *array.get_ptr(2));
   const dynamic& carray = array;