Remove the assert on get_register

[hugsy]

Remove the assert on get_register

assert are bad, so we remove one here, and give a second chance for get_register to execute properly if/when register doesn't start with a $ (like PPC or MIPS at least).

Description/Motivation/Screenshots

Register names don't always start with $ (sometimes nothing - MIPS/PPC, something $ - x64, sometimes % - SPARC).
This PR allows get_register to still attempt to get the value using Frame.read_register() even if the register doesn't start with $.

This PR should fix #752

How Has This Been Tested?

Architecture	Yes/No	Comments
x86-32	✔️
x86-64	✔️
ARM	✔️
AARCH64	✔️
MIPS	✔️
POWERPC	✔️
SPARC	❓
RISC-V	❓

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• My change adds tests as appropriate.
• I have read and agree to the CONTRIBUTING document.