don't closeMutex() the Capability lock

There might be threads in foreign calls that will attempt to return
via resumeThread() and grab this lock, so we can't safely destroy it.

Fixes one cause of

   internal error: ASSERTION FAILED: file Capability.c, line 90

although I haven't repeated that assertion failure in the wild, only
with a specially crafted test case, so I can't be sure I really got
it.