Fix off by one error in thrmgr error cleanup code
Fix off by one error in thrmgr error cleanup code

This should remove another potential source of segmentation faults when
the thread manager fails to start.

Issue:          #10
Reported by:    linuxton

heavilessrose/sslsplit,mimimitm/sslsplit,DJMIN/sslsplit,heavilessrose/sslsplit,mimimitm/sslsplit,DJMIN/sslsplit