Fix a crash on unsetting preset alias (re: ddaa145b) (#133)

The following set of commands caused ksh to crash:

$ unalias history; unalias r
Memory fault

When ksh is compiled with -D_std_malloc, the crash always
occurs when the 'r' alias is removed with 'unalias r',
although with vmalloc 'unalias history' must be run first
for the crash to occur. With the native malloc, the crash
message is also different:

$ unalias history; unalias r
free(): invalid pointer
Abort

This crash happens because when an alias is unset, _nv_unset
removes the NV_NOFREE flag which results in an invalid use
of free(3) as nv_isattr no longer detects NV_NOFREE afterward.
The history and r aliases shouldn't be freed from memory by
nv_delete because those aliases are given the NV_NOFREE attribute.

src/cmd/ksh93/bltins/typeset.c:
- Save the state of NV_NOFREE for aliases to fix the crash
  caused by 'unalias r'.

src/cmd/ksh93/tests/alias.sh:
- Use unalias on both history and r to check for the crash.
  'unalias -a' can't be used to replicate the crash.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/tests/alias.sh

@@ -109,9 +109,9 @@ alias foo=bar
 unalias foo
 unalias foo && err_exit 'unalias should return non-zero when a previously set alias is unaliased twice'
 
-# Removing a preset alias should work without an error from free(3)
-err=$(set +x; { "$SHELL" -i -c 'unalias history'; } 2>&1) && [[ -z $err ]] \
-|| err_exit "removing a preset alias does not work (got $(printf %q "$err"))"
+# Removing the history and r aliases should work without an error from free(3)
+err=$(set +x; { "$SHELL" -i -c 'unalias history; unalias r'; } 2>&1) && [[ -z $err ]] \
+|| err_exit "the 'history' and 'r' aliases can't be removed (got $(printf %q "$err"))"
 
 # ======
 exit $((Errors<125?Errors:125))