gallium/osmesa: Set freed osmesa->current_buffer to NULL.
Fix defect reported by Coverity Scan.
Use after free (USE_AFTER_FREE) use_after_free: Using freed pointer osmesa->current_buffer.
Fixes: 26c6f6cf ("gallium/osmesa: Remove the broken buffer-reuse scheme.") Signed-off-by: Vinson Lee vlee@freedesktop.org
cc: @anholt