use the correct return values in Fuzz() functions

[marten-seemann]

The docs say:

Data is a random input generated by go-fuzz, note that in most cases it is invalid. The function must return 1 if the fuzzer should increase priority of the given input during subsequent fuzzing (for example, the input is lexically correct and was parsed successfully); -1 if the input must not be added to corpus even if gives new coverage; and 0 otherwise; other values are reserved for future use.

[codecov]

Codecov Report

Merging #2705 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2705   +/-   ##
=======================================
  Coverage   86.77%   86.77%           
=======================================
  Files         124      124           
  Lines        9893     9893           
=======================================
  Hits         8584     8584           
  Misses        979      979           
  Partials      330      330           

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