fix cdg null deref

[jumormt]

fix Issue #1549

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@fc185be). Learn more about missing BASE report.
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
svf/lib/Util/CDGBuilder.cpp	0.00%	13 Missing ⚠️
svf/lib/Graphs/CDG.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1551   +/-   ##
=========================================
  Coverage          ?   62.67%           
=========================================
  Files             ?      245           
  Lines             ?    25707           
  Branches          ?     4502           
=========================================
  Hits              ?    16111           
  Misses            ?     9596           
  Partials          ?        0           

Files with missing lines	Coverage Δ
svf/include/Graphs/CDG.h	0.00% <ø> (ø)
svf/lib/Graphs/CDG.cpp	0.00% <0.00%> (ø)
svf/lib/Util/CDGBuilder.cpp	0.00% <0.00%> (ø)