Guard NULL from GetStringUTFChars in raiseTypeError

[SAY-5]

Fixes #643. In raiseTypeError, the return value of GetStringUTFChars was passed straight to PyErr_Format as a %s argument. Per the JNI spec that call returns NULL if it can't allocate the native string, and formatting NULL with %s crashes. This falls back to "" for the message and skips the matching ReleaseStringUTFChars when nothing was returned, so the type-error path stays intact on that rare allocation failure.

[bsteffensmeier]

It appears there are 3 possible failure modes a JVM could use when GetStringUTFChars fails.

1. It can exit the JVM or crash
2. It can return null with no exception
3. It can return null and raise an OutOfMemoryError

There is nothing we can do in the first case. This fix is good for the second case. In the third case having a java exception set is likely to cause problems later. Since this new code handles the exception I think we need to add a call to clear the JNI exception.

[SAY-5]

Good catch on the OutOfMemoryError case. I added an ExceptionClear right after the failed GetStringUTFChars so the pending JNI exception doesn't leak into later calls; the fallback name still gets reported.

[bsteffensmeier]

Looks good to me, thanks