'FormatTest' Unit Tests crash when run under GC

[tkarpinski]

Environment:
Xcode: 4.0.2
Mac OS: 10.6.7

When running the unit tests with the -fobjc-gc switch, the 'FormatTest' suite crashes with an EXC_BAD_ACCESS.

Test Suite 'FormatTest' started at 2011-05-23 15:56:03 -0400
Test Case '-[FormatTest testData]' started.
Test Case '-[FormatTest testData]' passed (0.023 seconds).
Test Case '-[FormatTest testString]' started.
EXC_BAD_ACCESS: Line 143 in SBJsonStreamWriter.m

The violations always point back to SBStackState.m. I've rewritten SBStackState.m to use an NSMutableArray which is probably less performant but passes the Unit Tests in both GC and non-GC environments.

Here's the commit on my branch for discussion: d94a98a57262d8acc8955bca777d3f72449b35b4

I discovered this issue while tracking down a memory leak in GC with the SBStackState object - a finalize method that calls free(_stack) is required to not leak the object.