Fix off-by-one bug in wxVector::reverse_iterator::base()

This is embarrassing, but the iterator returned by this method seems to
have always been wrong, ever since it was added back in 946954d3bf
(Added reverse iterator to wxVector<T>, 2008-09-16). Moreover, it was
also broken in its const_reverse_iterator counterpart where it was
copy-and-pasted in f7ef20685f (Add wxVector<>::const_reverse_iterator,
2013-05-08).

Finally fix this to return the correct value and add at least a simple
unit test check that this method behaves as expected.

tests/vectors/vectors.cpp

@@ -334,6 +334,10 @@ TEST_CASE("wxVector::reverse_iterator", "[vector][reverse_iterator]")
     CHECK( ri < re );
     CHECK( ri <= re );
 
+    CHECK( rb.base() == v.end() );
+    CHECK( re.base() == v.begin() );
+    CHECK( *ri.base() == 9 );
+
 #if wxUSE_STD_CONTAINERS_COMPATIBLY
     std::vector<int> stdvec(rb, re);
     REQUIRE( stdvec.size() == 10 );