linux-user/main: Check errno when getting AT_EXECFD

linux-user/main: Check errno when getting AT_EXECFD

It's possible for AT_EXECFD to end up with a valid value of 0. Check
errno when using qemu_getauxval instead of return value to handle this
case.

Not handling this case leads to a confusing condition where the
executable ends up as fd 0, i.e. stdin.

Signed-off-by: Vivian Wang <uwu@dram.page>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Fixes: 0b959cf5e4cc ("linux-user: Use qemu_getauxval for AT_EXECFD")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2448
Message-ID: <20240723100545.405476-3-uwu@dram.page>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>