nsf.h
- In Tcl 8.6.1 it might be that a Tcl_Obj has length > 0 but bytes == NULL. We have
  to relax out tcl_obj-sanity test for this case to avoid false-positives from assertions()
- improve cppcheck flags
nsf.c:
- remove redundant definition
- reduce variable scope
- make sure to follow nonnull assumptions