altoslib: Check for null state.gps before accessing it in eeprom records

Used to be we'd set state.gps to garbage before seeing the first GPS
record; now we leave it null, which will cause crashes for code that
doesn't expect it. The code for reading and replaying eeprom data was
not checking and was nicely crashing as a result.

Signed-off-by: Keith Packard <keithp@keithp.com>