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Fix undefined behavior in ares_buf_replace() pointer arithmetic (#1099)

Fix https://github.com/c-ares/c-ares/issues/1098:

The expression `ptr - (size_t)(buf->alloc_buf + buf->offset)` casts a
pointer to size_t then subtracts it from another pointer, which is
undefined behavior in C. UBSan flags this on every ares_buf_replace()
call as "applying non-zero offset to non-null pointer produced null
pointer".

Change to `ptr - buf->alloc_buf - buf->offset` which computes the same
offset using well-defined pointer-integer subtraction.

Signed-off-by: Kaixuan Li (@MarkLee131)

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