Fixed string_leak

[astigsen]

This fixes the leak from the string column as exposed by the AdaptiveStringLeak unit-test.

@rrrlasse @kspangsege

[kspangsege]

Ok, I'll rerun the part of the test suite that originally exposed the leak and let you know if that turns up anything.

[kspangsege]

Yep, that fixes all remaining leaks from a full duration test suite run on Linux.

valgrind --quiet --error-exitcode=1 --track-origins=yes --leak-check=yes --leak-resolution=low ./tightdb-tests-dbg --no-error-exitcode
Running Debug unit tests
TIGHTDB_MAX_LIST_SIZE = 4
Compiler supported SSE (auto detect): Yes
This CPU supports SSE (auto detect):  4.2

large_tests/test_strings.cpp:43: Begin ColumnString_monkeytest2
Success: 1 tests passed.
Test time: 7h57m

Top 5 time usage:
ColumnString_monkeytest2:       7h57m

[kspangsege]

👍

[kspangsege]

Alexander, PR's that are to be part of the next release must target the 'release_v0.1.1' branch instead of 'master'. In this case, it's easy to fix, because 'master' still doesn't contain anything that is not going into the release. I'll make sure this particular PR is also merged into 'release_v0.1.1', but we all have to learn to be strict about choosing the right target branch (I'll gladly elaborate on why this is important, if you want me to).

By the way, consider adding your thumbs-up to this one:

isaacs/github#18