Avoid -Wstringop-overflow= warning in iconv module.

Avoid -Wstringop-overflow= warning in iconv module.

On s390x when compiling with GCC 12, I get this warning:
utf8-utf16-z9.c:
../iconv/loop.c: In function ‘__from_utf8_loop_etf3eh_single’:
../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
  445 |     bytebuf[inlen++] = *inptr++;
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../iconv/loop.c:381:17: note: at offset 4 into destination object ‘bytebuf’ of size 4
  381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
      |                 ^~~~~~~
../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
  445 |     bytebuf[inlen++] = *inptr++;
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../iconv/loop.c:381:17: note: at offset 5 into destination object ‘bytebuf’ of size 4
  381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
      |                 ^~~~~~~

This patch tells the compiler that inend is always behind inptr which
avoids the warning.  Note that the SINGLE function is only used to
implement the mb*towc*() or wc*tomb*() functions.  Those functions use
inptr and inend pointing to a variable on stack, compute the inend pointer
or explicitly check the arguments which always leads to inptr < inend.

Special notes for backporters (according to Siddhesh Poyarekar):
If someone wants to backport this patch to release branches, they should
also backport the following wcrtomb change. Otherwise the assumptions
assumed by this patch are not true.

commit 9bcd12d223a8990254b65e2dada54faa5d2742f3
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Fri May 13 19:10:15 2022 +0530

    wcrtomb: Make behavior POSIX compliant

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>