librz/core: Fix disassembly (e.g. pd) with unmapped gaps

[thestr4ng3r]

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 every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
• I've updated the Rizin book with the relevant information (if needed).

Detailed description

Regression from 3b7f273:
rz_io_nread_at stops at unmapped regions, but disassembly being a
user-visible display needs to show gaps as 0xff and mapped regions after
with their contents again instead of just breaking and showing io->Oxff.
The exact meaningd of rz_io_nread_at and rz_io_read_at_mapped have also
been clarified in documentation comments and tests.

Test plan

$ rz -a arm -b 64
[0x00000000]> o malloc://1024 0x10
[0x00000000]> pd 4 @ 0x10
            0x00000010      00000000       udf   0
            0x00000014      00000000       udf   0
            0x00000018      00000000       udf   0
            0x0000001c      00000000       udf   0
[0x00000000]> pd 4 @ 0xc
            0x0000000c      ffffffff       invalid
            0x00000010      ffffffff       invalid
            0x00000014      ffffffff       invalid
            0x00000018      ffffffff       invalid

Notice how in the second pd, all bytes starting at 0x00000010 are read incorrectly.

After:

$ rz -a arm -b 64
[0x00000000]> o malloc://1024 0x10
[0x00000000]> pd 4 @ 0x10
            0x00000010      00000000       udf   0
            0x00000014      00000000       udf   0
            0x00000018      00000000       udf   0
            0x0000001c      00000000       udf   0
[0x00000000]> pd 4 @ 0xc
            0x0000000c      ffffffff       invalid
            0x00000010      00000000       udf   0
            0x00000014      00000000       udf   0
            0x00000018      00000000       udf   0

[Rot127]

Just this one nitpick. Otherwise feel free to merge.

[Rot127]

Suggested change

	ut8 *buf = calloc(1, n_bytes + 1);
	ut8 *buf = RZ_NEWS0(ut8, n_bytes + 1);

More a fan of RZ_NEWS0.
Mostly because we encourage new comers to use our macros and not calloc