folly/futures/detail/Core.h: avoid FP from clang's -Wconditional-uninitialized

Summary:
Avoid this false positive from clang:

  folly/futures/detail/Core.h:369:15: error: variable 'priority' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]

Reviewed By: yfeldblum

Differential Revision: D5369916

fbshipit-source-id: 49649417d1b799cc3945cba456b42ab438ad2ddc

folly/futures/detail/Core.h

@@ -325,7 +325,8 @@ class Core final {
 
   void doCallback() {
     Executor* x = executor_;
-    int8_t priority;
+    // initialize, solely to appease clang's -Wconditional-uninitialized
+    int8_t priority = 0;
     if (x) {
       if (!executorLock_.try_lock()) {
         executorLock_.lock();