egl/dri2: silence uninitialized variable warnings

And update assertions to be more informative.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
diff --git a/src/egl/drivers/dri2/egl_dri2.c b/src/egl/drivers/dri2/egl_dri2.c
index fe5cbc8..8b915ef 100644
--- a/src/egl/drivers/dri2/egl_dri2.c
+++ b/src/egl/drivers/dri2/egl_dri2.c
@@ -1256,7 +1256,8 @@
       format = __DRI_TEXTURE_FORMAT_RGBA;
       break;
    default:
-      assert(0);
+      assert(!"Unexpected texture format in dri2_bind_tex_image()");
+      format = __DRI_TEXTURE_FORMAT_RGBA;
    }
 
    switch (dri2_surf->base.TextureTarget) {
@@ -1264,7 +1265,8 @@
       target = GL_TEXTURE_2D;
       break;
    default:
-      assert(0);
+      target = GL_TEXTURE_2D;
+      assert(!"Unexpected texture target in dri2_bind_tex_image()");
    }
 
    (*dri2_dpy->tex_buffer->setTexBuffer2)(dri2_ctx->dri_context,