Handle scenario when None is passed as memory limit

[rabsr]

Fixes #1117

[codecov]

Codecov Report

Merging #1118 (c29ebcb) into development (f8bab15) will decrease coverage by 0.54%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #1118      +/-   ##
===============================================
- Coverage        85.49%   84.94%   -0.55%     
===============================================
  Files              137      137              
  Lines            10557    10557              
===============================================
- Hits              9026     8968      -58     
- Misses            1531     1589      +58     

Impacted Files	Coverage Δ
autosklearn/automl.py	84.61% <100.00%> (-0.15%)	⬇️
autosklearn/smbo.py	74.07% <0.00%> (-9.77%)	⬇️
...eline/components/feature_preprocessing/fast_ica.py	91.30% <0.00%> (-6.53%)	⬇️
autosklearn/util/pipeline.py	76.56% <0.00%> (-6.25%)	⬇️
..._preprocessing/select_percentile_classification.py	86.20% <0.00%> (-3.45%)	⬇️
autosklearn/evaluation/__init__.py	77.35% <0.00%> (-3.42%)	⬇️
...osklearn/metalearning/metafeatures/metafeatures.py	93.34% <0.00%> (-1.37%)	⬇️
...ine/components/classification/gradient_boosting.py	91.30% <0.00%> (-0.87%)	⬇️
autosklearn/estimators.py	92.30% <0.00%> (-0.77%)	⬇️

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[rabsr]

Not sure why test cases are failing. Looks like there is some issue in fetching data from openml

[mfeurer]

Looks reasonable, thanks a lot!