Restore #22 'unset -f' fix minus segfault (re: b7932e87, 97511748)

Applying the fix for 'unset -f' exposed a crashing bug in lookup()
in sh/nvdisc.c, which is the function for looking up discipline
functions. This is what caused tests/variables.sh to crash.
Ref.: https://github.com/ksh93/ksh/issues/23#issuecomment-645699614

src/cmd/ksh93/sh/nvdisc.c: lookup():
- To avoid segfault, check that the function pointer nq->nvalue.rp
  is actually set before checking if nq->nvalue.rp->running==1.

src/cmd/ksh93/sh/xec.c,
src/cmd/ksh93/tests/functions.sh:
- Uncomment the 'unset -f' fix from b7932e87.

Resolves #21 (again).

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-17:
+
+- A bug in 'unset -f' was fixed that prevented shell functions from
+  unsetting themselves while they were running. A POSIX function no longer
+  crashes when doing so, and a KornShell-style function no longer silently
+  ignores an 'unset -f' on itself. A function of either form now continues
+  running after unsetting itself, and is removed at the end of the run.
+
 2020-06-16:
 
 - Passing the '-d' flag to the read builtin will no longer cause the '-r'