Cursor::read<T>() initializes return value

Summary:
If the compiler doesn't see thru the call to tryRead which always initializes the val variable, it will emit the following error.
I was getting error when compiling my android project:
error: 'val' may be used uninitialized in this function [-Werror=maybe-uninitialized]
This should not have a negative perf impact as smart compilers which inline call to tryRead can elliminate the initialization.

Reviewed By: djwatson

Differential Revision: D4790864

fbshipit-source-id: f353cfe54ca4d056b6ddfc075d00580c9f2d391e

folly/io/Cursor.h

@@ -227,7 +227,7 @@ class CursorBase {
 
   template <class T>
   T read() {
-    T val;
+    T val{};
     if (!tryRead(val)) {
       std::__throw_out_of_range("underflow");
     }