Empty clean call leads to memory leak

Instrumenting an empty clean call (without any instrs) using
```
dr_insert_clean_call(drcontext, bb, inst, (void *)empty_clean_call, false, 0);
```
leads to a memory leak on x86.

Discovered this while working on a PR, will add a link when I push it.

Start by reproducing the leak on x86 with the empty dr_insert_clean_call invocation shown in the issue, using empty_clean_call as the callback. Trace the clean-call insertion and cleanup paths to identify the allocation that remains live; done means the empty call no longer leaks memory.

Written by the indexing model from the issue text.