Check array get for < 0

Summary: dynamic's integer type is signed, so make sure array indices
are not negative.

(See https://our.intern.facebook.com/intern/tasks/?t=7445055)

Reviewed By: @Gownta

Differential Revision: D2145689

folly/dynamic.cpp

@@ -185,7 +185,7 @@ dynamic const& dynamic::at(dynamic const& idx) const {
     if (!idx.isInt()) {
       throw TypeError("int64", idx.type());
     }
-    if (idx >= parray->size()) {
+    if (idx < 0 || idx >= parray->size()) {
       throw std::out_of_range("out of range in dynamic array");
     }
     return (*parray)[idx.asInt()];

folly/test/DynamicTest.cpp

@@ -157,6 +157,7 @@ TEST(Dynamic, ArrayBasics) {
   EXPECT_EQ(array.at(1), 2);
   EXPECT_EQ(array.at(2), 3);
 
+  EXPECT_ANY_THROW(array.at(-1));
   EXPECT_ANY_THROW(array.at(3));
 
   array.push_back("foo");