rigol-ds: free memory that was allocated by SCPI get routines

rigol-ds: free memory that was allocated by SCPI get routines

The SCPI get routines may allocate memory for response data which
callers have to free after use.

The approach to release allocated memory is simlar or identical to the
corresponding parts of pull request 95, in an attempt to get the reported
leak fixes in, yet reduce the potential for conflicts where this commit
deviates from the PR. Most work was done by Ralf. Thank you!

This addresses part of bug #1683.

Submitted-By: Reported-By: Ralf <redacted>
Tested-By: Reported-By: Ralf <redacted>