Fix for stacking mixed output redirects.

[karliss]

That is redirect to file + redirect to buffer which happens when calling rz_core_cmd_str .

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

Problem discovered while investigating rizinorg/cutter#3426 but as shown by testcase it's possible to trigger it purely within rizin as well.

In case of cutter it could be simply triggered by running cmd > outputfile in console widget.

Test plan

In addition to command test, I also tested the patch fixes basic redirect case in Cutter.

Closing issues

[notxvilka]

Good finding!