st/mesa: fix handling the fallback texture

This fixes crash when post-processing is enabled in SW:KotOR.

v2: fix const-ness
v3: move assignment into the if() block

Signed-off-by: Miklós Máté <mtmkls@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
1 file changed