Fix loading a saved recording

Fix loading a saved recording

Currently you get this assertion failure if you try to execute the
inferior after loading a saved recording, when no recording was done
earlier in the same gdb session:
```
$ gdb -q c -ex "record restore test.rec"
Reading symbols from c...
[New LWP 26428]
Core was generated by `/tmp/c'.
Restored records from core file /tmp/test.rec.
(gdb) c
Continuing.
../../gdb/inferior.c:293: internal-error: inferior* find_inferior_pid(process_stratum_target*, int): Assertion `pid != 0' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
```

The change in step-precsave.exp triggers this bug, since now the
recording is loaded in a new gdb session, where
record_full_resume_ptid was never set.

The fix is to simply set record_full_resume_ptid when resuming a loaded
recording.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31971
Approved-By: Guinevere Larsen <blarsen@redhat.com>