st/mesa: release EGLImage on EGLImageTarget* error

st/mesa: release EGLImage on EGLImageTarget* error

The smapi->get_egl_image() call in st_egl_image_get_surface() stores a
reference to the EGLImage's texture in stimg.texture. That reference is
released via pipe_resource_reference(&stimg.texture, NULL) before stimg
goes out of scope at the end of the function, but not in the error path
if !is_format_supported().

Fixes: 83e9de25f325 ("st/mesa: EGLImageTarget* error handling")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>