scpi_vxi: fix memory leak for SCPI response data in VXI support code

scpi_vxi: fix memory leak for SCPI response data in VXI support code

Routine scpi_vxi_read_data() invokes device_read_1(), which provides a
static buffer where dynamically allocated memory for SCPI response data
is kept. Release this memory after getting a copy of the response data,
before the next device_read_1() call loses the reference.

Valgrind stats without the fix:

  ==238825== LEAK SUMMARY:
  ==238825== definitely lost: 45,547,737 bytes in 18,331 blocks
  ==238825== indirectly lost: 0 bytes in 0 blocks
  ==238825== possibly lost: 48,154 bytes in 14 blocks
  ==238825== still reachable: 42,859 bytes in 288 blocks
  ==238825== suppressed: 0 bytes in 0 blocks

Valgrind stats with the fix:

  ==239413== LEAK SUMMARY:
  ==239413== definitely lost: 40 bytes in 2 blocks
  ==239413== indirectly lost: 0 bytes in 0 blocks
  ==239413== possibly lost: 0 bytes in 0 blocks
  ==239413== still reachable: 38,613 bytes in 287 blocks
  ==239413== suppressed: 0 bytes in 0 blocks

Remaining leaks in scpi_vxi_open() are of lesser severity because they
don't accumulate during execution.

[ gsi: rebase to recent master, reword commit message ]