wide screen age picker can hang shell startup

[quozl]

When the screen is not very wide, the age picker shows a default already selected, and pressing enter works fine.

When the screen is wide, (1366x768 pixels), the age picker shows no default, and pressing enter clears the screen and the shell does not start.

shell.log shows a traceback relating to the age picker.

Workaround is to restart lightdm and pick an age properly.

[quozl]

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/jarabe/intro/window.py", line 438, in _create_profile_cb
    create_profile(user_profile)
  File "/usr/lib/python2.7/dist-packages/jarabe/intro/window.py", line 64, in create_profile
    agepicker.save_age(user_profile.age)
  File "/usr/lib/python2.7/dist-packages/jarabe/intro/agepicker.py", line 111, in save_age
    birth_timestamp = calculate_birth_timestamp(age)
  File "/usr/lib/python2.7/dist-packages/jarabe/intro/agepicker.py", line 46, in calculate_birth_timestamp
    age_in_seconds = age * _SECONDS_PER_YEAR
TypeError: unsupported operand type(s) for *: 'NoneType' and 'float'

[walter]

Not sure how age could ever be None, but:

​https://github.com/sugarlabs/sugar/commit/2a370a7f6ca82fac24b8ea9858fed9d3fb174b02 is a possible remedy.

[godiard]

Proposed a different solution ​https://github.com/sugarlabs/sugar/pull/507

[Gonzalo Odiard]

Age picker: check if is in a valid state - Fixes #4850

When the user press Return, we need check if the page is in a valid state
before checking if is the last page.

Changeset: eb0615b365b944b32efa8996f95a6989fcc66cd9