main/texobj: Check that texture id > 0 before looking it up in hash-table

_mesa_lookup_texture_err() is not currently checking that the
texture-id can be zero, but _mesa_HashLookup() doesn't expect the key
to be zero, and will fail an assertion.

Considering that _mesa_lookup_texture_err() is called from
_mesa_GetTextureImage and _mesa_GetTextureSubImage with user provided
arguments, we must validate the texture-id before looking it up in the
hash-table.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/mesa/main/texobj.c

@@ -116,9 +116,10 @@ _mesa_lookup_texture(struct gl_context *ctx, GLuint id)
 struct gl_texture_object *
 _mesa_lookup_texture_err(struct gl_context *ctx, GLuint id, const char* func)
 {
-   struct gl_texture_object *texObj;
+   struct gl_texture_object *texObj = NULL;
 
-   texObj = _mesa_lookup_texture(ctx, id); /* Returns NULL if not found. */
+   if (id > 0)
+      texObj = _mesa_lookup_texture(ctx, id); /* Returns NULL if not found. */
 
    if (!texObj)
       _mesa_error(ctx, GL_INVALID_OPERATION, "%s(texture)", func);