Fix PBKDF2 with zero iterations

[randombit]

It would go into a very long loop.

OpenSSL treats iterations==0 same as iterations==1 but this seems confusing. Instead just reject it.

Unrelated, fix a divide by zero if asked to tune with 0 byte output.

Closes GH #2088

[codecov-io]

Codecov Report

Merging #2090 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2090      +/-   ##
==========================================
- Coverage   92.38%   92.37%   -0.01%     
==========================================
  Files         545      545              
  Lines       59257    59261       +4     
  Branches     6241     6243       +2     
==========================================
+ Hits        54742    54743       +1     
- Misses       4515     4518       +3

Impacted Files	Coverage Δ
src/lib/pbkdf/pbkdf2/pbkdf2.cpp	84.09% <50%> (-1.63%)	⬇️
src/lib/pk_pad/emsa_raw/emsa_raw.cpp	71.42% <0%> (-2.86%)	⬇️

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