Modules: Fix maxnetblock size check in several modules

[bcoles]

In some modules max_netblock is defined then subsequently overwritten with a potentially invalid value.

[codecov-commenter]

Codecov Report

Merging #1701 (b9b0921) into master (368fe2b) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1701   +/-   ##
=======================================
  Coverage   46.47%   46.48%           
=======================================
  Files         709      709           
  Lines       46758    46754    -4     
=======================================
  Hits        21733    21733           
+ Misses      25025    25021    -4     

Impacted Files	Coverage Δ
modules/sfp_alienvault.py	19.69% <ø> (+0.07%)	⬆️
modules/sfp_fraudguard.py	25.00% <0.00%> (+0.19%)	⬆️
modules/sfp_pulsedive.py	25.98% <ø> (+0.20%)	⬆️
modules/sfp_robtex.py	17.46% <ø> (+0.13%)	⬆️

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