AP_Camera: avoid SITL panic if pic taken with no pos estimate

[rmackay9]

This resolves issue #29530 by checking that the current_loc has been initialised before attempting to get the altitude from it.

This fix is thanks to @blaab2 and the commit is co-authored with him, thanks!

I've lightly tested this in SITL by performing a before/after test to confirm the issue occurs before this fix and does not afterwards.

BTW, the issue can be reproduced in SITL by doing this:

1. rm eeprom.bin file
2. start SITL
3. param set LOG_DISARMED 1
4. param set GPS1_TYPE 0
5. restart SITL
6. right-mouse-button click on MP's map and select, "Trigger Camera NOW"

Below is a screenshot of what appears in the LOGS which is a mostly zero message

[peterbarker]

LGTM