Fix exception in MPFuture.__del__()

[justheuristic]

This PR addresses the bug reported in #552 - or, at least, it should, since we cannot reproduce the problem locally.

[codecov]

Codecov Report

Merging #555 (49e4569) into master (7d1bb7d) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   75.90%   75.98%   +0.07%     
==========================================
  Files          81       81              
  Lines        7995     7995              
==========================================
+ Hits         6069     6075       +6     
+ Misses       1926     1920       -6     

Impacted Files	Coverage Δ
hivemind/utils/mpfuture.py	89.33% <100.00%> (ø)
hivemind/dht/protocol.py	93.15% <0.00%> (+0.45%)	⬆️
hivemind/moe/server/server.py	44.26% <0.00%> (+0.54%)	⬆️
hivemind/dht/routing.py	94.11% <0.00%> (+0.58%)	⬆️
hivemind/moe/server/connection_handler.py	47.91% <0.00%> (+1.04%)	⬆️
hivemind/moe/server/runtime.py	70.00% <0.00%> (+1.66%)	⬆️