mesa: fix refcnt imbalance related to egl_image_target_texture()

Indeed, the locally allocated "stimg" reference was not freed on a specific code path.

For instance, this issue is triggered on radeonsi or r600 with: "piglit/bin/egl-ext_egl_image_storage -auto -fbo" while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: 6a3f5c65 ("mesa: simplify st_egl_image binding process for texture storage") Signed-off-by: Patrick Lerda patrick9876@free.fr