libstdc++: Fix std::deque::size() Xmethod [PR112491]

The Xmethod for std::deque::size() assumed that the first element would
be at the start of the first node. That's only true if elements are only
added at the back. If an element is inserted at the front, or removed
from the front (or anywhere before the middle) then the first node will
not be completely populated, and the Xmethod will give the wrong result.

libstdc++-v3/ChangeLog:

	PR libstdc++/112491
	* python/libstdcxx/v6/xmethods.py (DequeWorkerBase.size): Fix
	calculation to use _M_start._M_cur.
	* testsuite/libstdc++-xmethods/deque.cc: Check failing cases.