fix crash when when running `analyze` with `-only`. 

This fixes the bug @yiding noticed --

```
2013-08-29 14:09:57.074 xctool[63537:2507] -[Buildable objectForKeyedSubscript:]: unrecognized selector sent to instance 0x7ff3e3330a90

2013-08-29 14:09:57.210 xctool[63537:2507] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Buildable objectForKeyedSubscript:]: unrecognized selector sent to instance 0x7ff3e3330a90'
*** First throw call stack:
(
0   CoreFoundation                      0x00007fff984aab06 __exceptionPreprocess + 198
1   libobjc.A.dylib                      0x00007fff97bb13f0 objc_exception_throw + 43
2   CoreFoundationtion                      0x00007fff9854140a -[NSObject(NSObject) doesNotRecognizeSelector:] + 186
3   CoreFoundation                      0x00007fff984aab069902e ___forwarding___ + 414
4   CoreFoundation 0x00007fff984aab069902ex00007fff98498e18 _CF_forwarding_prep_0 + 232
5   xctool                                0x000000010f1c33e3 -[AnalyzeAction performActionWithOptions:xcodeSubjectInfo:] + 557
6   xctool                              0x000000010f1c33e30010f1abd12 -[XCTool run] + 1967
7   xctool                               0x000000010f1aadfe main + 514
8   xctool                               0x000000010f1aabf4 start + 52
)
libc++abi.dylib: terminate called throwing an exception
```