enter the regular run loop if no undecryptable packet was processed

[marten-seemann]

Fixes #3265.

[codecov]

Codecov Report

Merging #3268 (a54b00e) into master (ebcd98e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3268   +/-   ##
=======================================
  Coverage   85.47%   85.47%           
=======================================
  Files         132      132           
  Lines        9799     9799           
=======================================
  Hits         8375     8375           
  Misses       1048     1048           
  Partials      376      376           

Impacted Files	Coverage Δ
session.go	77.63% <100.00%> (ø)

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[paralin]

Tested-by: Christian Stewart <christian@paral.in>