Cygwin: normalize_win32_path: Avoid buffer underruns

Cygwin: normalize_win32_path: Avoid buffer underruns

Thanks to Ken Harris <Ken.Harris@mathworks.com> for the diagnosis.

When backing up tail to handle a "..", the code only checked that
it didn't underrun the destination buffer while removing path
components.  It did *not* take into account that the first backslash
in the path had to be kept intact.  Example path to trigger the
problem: "C:\A..\..\..\B'

Fix this by moving the dst pointer to the first backslash so subsequent
tests cannot underrun this position.  Also make sure that we always
*have* a backslash.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>