Fix in isoSpeed()

[piponazo]

Fix #561.

I am not very familiar with the code, but It seems that we were using the wrong counter for the idx variable in the isoSpeed() function. The counter variable cnt had a value of 25 while the number of keys is always a number between 1 and 3 (sensKeys->count).

[clanmills]

Never look at that code. However I think it's a fuzzed file that has a deliberately illegal value that causes an out-of-bouds reference in a static structure. If the POC now passes and the test suite is happy, this is almost certainly a good fix.

Thanks for dealing with this.

[codecov]

Codecov Report

Merging #564 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
+ Coverage   62.73%   62.74%   +<.01%     
==========================================
  Files         154      154              
  Lines       20680    20680              
==========================================
+ Hits        12974    12976       +2     
+ Misses       7706     7704       -2

Impacted Files	Coverage Δ
src/easyaccess.cpp	97.59% <100%> (+1.2%)	⬆️
src/bigtiffimage.cpp	62.03% <0%> (+0.53%)	⬆️

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