rts: Fix retainerProfile early return with TREC_CHUNK

In the retainer profiler when pop() returns with *c == NULL retainerProfile, more or less the top level function there, will immediately return.

All other code paths is pop() continue with the next stackElement when this happens. So it seems weird to me that TREC_CHUNK we would suddenly abort everything even though the stack might still have elements left to process.

See the attached one line patch :)

I'm not sure this is actually a bug since I don't really understand the TREC stuff. @simonmar you wrote this code >10 years ago, any comments? :)