Use blocksize to determine size for rz_cmd_disassembly_n_bytes_handler()

[winterheart]

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 documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Commands like pD are accepts optional argument, but rz_cmd_disassembly_n_bytes_handler() requires it. Add current core->blocksize as failback to help calculating correct size.

Test plan

Current dev threats no args pD as error, but rizin book and pD? states that should be correct command:

[0x00001100]> bf sym.main
[0x00001100]> pD @ sym.main
ERROR: Invalid number of arguments

After applied changes pD should correctly dump file without optional argument:

[0x00001100]> bf sym.main
[0x00001100]> pD @ sym.main
            ;-- main:
            0x000011e9      endbr64
            0x000011ed      push  rbp
            0x000011ee      mov   rbp, rsp
            0x000011f1      sub   rsp, 0x40
            0x000011f5      mov   dword [rbp - 0x34], edi
            0x000011f8      mov   qword [rbp - 0x40], rsi
...

Closing issues

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