aco: fix printing ASM on GFX6-7 again

Checking errno is actually wrong because it's only updated if
popen() fails (ie. NULL). One solution is to check if the first
line is empty.

Fixes: c95d258d1bc ("aco: fix printing ASM on GFX6-7 if clrxdisasm is not found")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5591>
1 file changed