folly: fix FutureDAGTest compilation

Summary:
```
folly/experimental/test/FutureDAGTest.cpp:46:7: error: ‘source_node’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       if (it->first != source_node) {
       ^~
cc1plus: all warnings being treated as errors
```

Reviewed By: yfeldblum

Differential Revision: D14702569

fbshipit-source-id: 949ecde007a8d84498c5d87f4a4c2db463ba83a9

folly/experimental/test/FutureDAGTest.cpp

@@ -30,7 +30,7 @@ struct FutureDAGTest : public testing::Test {
   }
 
   void reset() {
-    Handle source_node;
+    Handle source_node{0};
     std::unordered_set<Handle> memo;
     for (auto& node : nodes) {
       for (Handle handle : node.second->dependencies) {