fluke-45: 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.

This implementation is incomplete. The fluke-45 driver's context holds
a "global" copy of the most recently received response. While the data
is freed in the next receive call, one item remains allocated for the
driver's remaining life time. Which is considered non-critical.

Also moves an operator in a complex multi-line expression to a different
location to follow the project's style.

This addresses part of bug #1683.