client: fix minor prefs parsing bug

[davidpanderson]

If we parse for a target venue, and it's present, and mod_time/source_project/source_scheduler appear after the venue, we were failing to parse them.

Fixes #5682

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 10.53%. Comparing base (ab1085b) to head (2b3c19e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5704 +/- ## ============================================ - Coverage 10.53% 10.53% -0.01% Complexity 1068 1068 ============================================ Files 279 279 Lines 35874 35875 +1 Branches 8412 8412 ============================================ Hits 3780 3780 - Misses 31700 31701 +1 Partials 394 394 ``` | [Files](https://app.codecov.io/gh/BOINC/boinc/pull/5704?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BOINC) | Coverage Δ | | |---|---|---| | [lib/prefs.cpp](https://app.codecov.io/gh/BOINC/boinc/pull/5704?src=pr&el=tree&filepath=lib%2Fprefs.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BOINC#diff-bGliL3ByZWZzLmNwcA==) | `0.00% <0.00%> (ø)` | |

[bema-aei]

@davidpanderson Do I understand your fix correctly that whenever there is some global preference property (and tag) added that doesn't appear in the venues, there has to be code added here to treat that specifically (as it's currently done with source_project, source_scheduler, mod_time now)? I really don't know how likely such an addition is, though.

[davidpanderson]

All items need code to parse them. It's a question of where the code goes. See my patch.