Fix vfwscanf(3) assignment suppression flag handling bug

newlib's vfwscanf(3) (or specifically, __SVFWSCANF_R()) fails to correctly set
the assignment-suppressing character (`*') flag[1] which, when present in the
formatting string, results in undefined behaviour comprising retrieving and
dereferencing a pointer that was not supplied by the caller as such or at all.
When compared to the vfscanf(3) implementation, this would appear to be over
the missing goto match_failure statement preceded by the flags test seen below.
Hence, this patch (re)introduces it.

[1] <http://pubs.opengroup.org/onlinepubs/009695399/functions/fwscanf.html>

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