[PATCH] svfwscanf: Simplify _sungetwc_r to eliminate apparent buffer overflow

[Keith Packard]

svfwscanf replaces getwc and ungetwc_r. The comments in the code talk
about avoiding file operations, but they also need to bypass the
mbtowc calls as svfwscanf operates on wchar_t, not multibyte data,
which is a more important reason here; they would not work correctly
otherwise.

The ungetwc replacement has code which uses the 3 byte FILE _ubuf
field, but if wchar_t is 32-bits, this field is not large enough to
hold even one wchar_t value. Building in this mode generates warnings
about array overflow:

	In file included from ../../newlib/libc/stdio/svfiwscanf.c:35:
	../../newlib/libc/stdio/vfwscanf.c: In function '_sungetwc_r.isra':
	../../newlib/libc/stdio/vfwscanf.c:316:12: warning: array subscript 4294967295 is above array bounds of 'unsigned char[3]' [-Warray-bounds]
	  316 |   fp->_p = &fp->_ubuf[sizeof (fp->_ubuf) - sizeof (wchar_t)];
	      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	In file included from ../../newlib/libc/stdio/stdio.h:46,
			 from ../../newlib/libc/stdio/vfwscanf.c:82,
			 from ../../newlib/libc/stdio/svfiwscanf.c:35:
	../../newlib/libc/include/sys/reent.h:216:17: note: while referencing '_ubuf'
	  216 |   unsigned char _ubuf[3]; /* guarantee an ungetc() buffer */
	      |                 ^~~~~

However, the vfwscanf code *never* ungets data before the start of the
scanning operation, and *always* ungets data which matches the input
at that point, so the code always hits the block which backs up over
the input data and never hits the block which uses the _ubuf field.

In addition, the svfwscanf code will always start with the unget
buffer empty, so the ungetwc replacement never needs to support an
unget buffer at all.

Simplify the code by removing support for everything other than
backing up over the input data, leaving the check to make sure it
doesn't get underflowed in case the vfscanf code has a bug in it.

Signed-off-by: Keith Packard <keithp@keithp.com>

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-- 
-keith
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