Add $heap_base variable after running the heap command

[gsingh93]

This PR sets the $heap_base GDB variable once the heap command is run for the first time, allowing it to be used in later GDB commands or in GDB scripts.

Fixes #1049

[codecov-commenter]

Codecov Report

Merging #1051 (4d6eb0c) into dev (5e78222) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev    #1051      +/-   ##
==========================================
- Coverage   49.77%   49.77%   -0.01%     
==========================================
  Files         159      159              
  Lines       18653    18654       +1     
  Branches     1519     1519              
==========================================
  Hits         9285     9285              
- Misses       9068     9069       +1     
  Partials      300      300              

Impacted Files	Coverage Δ
pwndbg/commands/heap.py	45.46% <0.00%> (-0.07%)	⬇️

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[disconnect3d]

@gsingh93 We should probably add a tip on that and document it somehow e.g. in FEATURES.md :P

[CptGibbon]

I really like this feature and will be using it a lot ❤️
I think it could be improved with a couple of small changes:

• Check if the user has already set a convenience variable of the same name, either with init-if-undefined or gdb.convenience_variable("heap_base"). This way we won't break user scripts.
• Consider using gdb.set_convenience_variable("heap_base", <value>) in place of gdb.execute()
• Perhaps $heap_base could be populated on heap initialization, rather than after running one specific heap command

Thanks for this PR!

[gsingh93]

Thanks for the feedback @CptGibbon. Those are all great suggestions, but I'm not sure when I'll get time to make those changes. For now I've filed these suggestions as a new issue so they don't get lost: #1060