make io::Cursor::push() safe to call with an empty ByteRange

make io::Cursor::push() safe to call with an empty ByteRange

Summary:
Clang's UndefinedBehaviorSanitizer flagged an issue that pushAtMost() could
call `memcpy(dest, nullptr, 0)` if the input was an empty `ByteRange`.  A
default constructed `ByteRange` (or `StringPiece`) will be empty and both its
begin and end pointers will be null.  Unfortunately it is undefined behavior to
call `memcpy()` with a null source pointer even if the length is 0.

This updates the `Cursor` and `Appender` code to avoid calling `memcpy()` at
all when the input length is 0.

Reviewed By: yfeldblum

Differential Revision: D5322917

fbshipit-source-id: 67fce9579f97e7e93a5767b11cc5e43ff7be5876