Similar to BZ#19387, BZ#21014, and BZ#20971, both x86 sse2 strncat
optimized assembly implementations do not handle the size overflow
correctly.
The x86_64 one is in fact an issue with strcpy-sse2-unaligned, but
that is triggered also with strncat optimized implementation.
This patch uses a similar strategy used on 3daef2c8ee, where
saturared math is used for overflow case.
Checked on x86_64-linux-gnu and i686-linux-gnu. It fixes BZ #19390.
[BZ #19390]
* string/test-strncat.c (test_main): Add tests with SIZE_MAX as
maximum string size.
* sysdeps/i386/i686/multiarch/strcat-sse2.S (STRCAT): Avoid overflow
in pointer addition.
* sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S (STRCPY):
Likewise.
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