fix(windows): Reverse order of backspaces

In order for the BK_SURROGATE and BK_DEADKEY flags to be matched with
the correct xstring elements in the context, we need to delete the
elements from the buffer in reverse order.

decxstr would previously assert if attempting to move before start of
string. This is undesirable behaviour as it should instead return NULL
to indicate that we've finished the string.

I also corretected a buffer underrun which would be possible if
malformed data was in the xstring.

I checked all uses of decxstr to ensure that the NULL return value is
being tested correctly.