Problem: There is no check for buffer validity before processing
semantic tokens response. This can lead to `Invalid buffer id` error
if processing request takes a long time and the buffer is wiped out.
For example, this can happen after by accident navigating to a buffer
from different project which leads to first loading project's
workspace and *then* processing semantic tokens. During that time
a buffer can be wiped out, as navigation to it was by accident.
Solution: Add extra check for buffer validity before processing semantic
tokens response.
(cherry picked from commit a9cdf76e3a)
c40057f372