[GP6] Corruption when the gp file contains incomplete measures

3c7a69d / Windows7

1) Load this file created with GP6: test file1.gpx Another one: NG3.gpx
Result: corruption

The cause is due to the possibility - inadvertently, or otherwise - to save via GP a score with incomplete measures (see image below)

2) Load this other file: test file2.gpx

In this case, there is no corruption, because the first measure, incomplete, is automatically considered as a pickup measure.

- By exporting these files with the GP5 format, everything goes well. See: export.gp5

(so only an issue with GP6)