core/cmd_debug: fix OOB read when running 'dcu'

[ret2libc]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the documentation and the rizin book with the relevant information (if needed)

Detailed description

OOB read because cmd_dcu always looks for the argument. This will be really fixed once dcu is converted to newshell, but in the meantime...

Test plan

...

Closing issues

...

[codecov]

Codecov Report

Merging #745 (991d7d1) into dev (8a69088) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #745      +/-   ##
==========================================
+ Coverage   42.79%   42.82%   +0.03%     
==========================================
  Files         871      871              
  Lines      316551   316554       +3     
==========================================
+ Hits       135463   135576     +113     
+ Misses     181088   180978     -110     

Impacted Files	Coverage Δ
librz/core/cmd_debug.c	27.28% <100.00%> (-0.07%)	⬇️
librz/io/p/io_gdb.c	35.96% <0.00%> (-5.89%)	⬇️
librz/bp/bp_io.c	88.46% <0.00%> (-3.85%)	⬇️
shlr/gdb/src/packet.c	55.95% <0.00%> (-1.20%)	⬇️
shlr/gdb/src/gdbclient/xml.c	63.30% <0.00%> (-0.97%)	⬇️
librz/debug/debug.c	54.41% <0.00%> (-0.46%)	⬇️
librz/socket/socket.c	23.95% <0.00%> (-0.35%)	⬇️
librz/util/sys.c	60.54% <0.00%> (-0.23%)	⬇️
shlr/gdb/src/gdbclient/core.c	39.47% <0.00%> (+1.96%)	⬆️
... and 4 more

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