fuzz: return nil if len(keys) is zero

[odeke-em]

Fixes a divide by zero exception which resulted
from a modulo operation using keys that were non-existent.
oss-fuzz reported a crash

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=37182

but that was a fault in the fuzzing code, and not in smt.

[odeke-em]

/cc @cuonglm @liamsi

[codecov-commenter]

Codecov Report

Merging #55 (23d38e6) into master (a8b51d5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   85.68%   85.68%           
=======================================
  Files           6        6           
  Lines         468      468           
=======================================
  Hits          401      401           
  Misses         39       39           
  Partials       28       28           

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[odeke-em]

Thank you @adlerjohn and @cuonglm for the review!