Meetings: Add validation for date/time inputs

[ocean90]

When you enter "20" in the time field the meetings list will produce a fatal error:

Uncaught exception 'Exception' with message 'DateTime::__construct(): Failed to parse time string (2016-05-02 20 GMT) at position 11 (2): Unexpected character' in /home/wporg/public_html/wp-content/plugins/wporg-meeting-posttype/wporg-meeting-posttype.php:119
Stack trace: #0 /home/wporg/public_html/wp-content/plugins/wporg-meeting-posttype/wporg-meeting-posttype.php(119): DateTime->__construct('2016-05-02 20 G...')
#1 [internal function]: Meeting_Post_Type->{closure}(Object(WP_Post), 0)
#2 /home/wporg/public_html/wp-content/plugins/wporg-meeting-posttype/wporg-meeting-posttype.php(139): array_walk(Array, Object(Closure))
#3 [internal function]: Meeting_Post_Type->meeting_set_next_meeting(Array, Object(WP_Query))
#4 /home/wporg/public_html/wp-includes/plugin.php(291): call_user_func_array(Array, Array)
#5 /home/wporg/public_html/wp-includes/query.php(3766): apply_filters_ref_array('the_posts', Array)
#6 /home/wporg/public_html/wp-includes/query.php(4010): WP_Query->get_posts()
#7 /home/wporg/public_html/wp-inc
Timestamp: Mon, 02 May 2016 21:12:48 +0000 (1462223568)
File: /home/wporg/public_html/wp-content/plugins/wporg-meeting-posttype/wporg-meeting-posttype.php
Line: 119

The error will be in the admin too so you have to fix the meta field via wp-cli.

[Otto42]

Fixed in [3173].