Fix restoring DEBUG trap upon exiting virtual subshell

This trap failed to be restored correctly when being trapped in
a subshell, causing corruption or a crash when restoring the
parent shell environment's trap upon leaving the subshell.

Thanks to Koichi Nakashima for the report and reproducer.

src/cmd/ksh93/sh/fault.c: sh_sigreset():
- Fix an off-by-one error in the loop that restores the
  pseudosignal traps.

src/cmd/ksh93/tests/basic.sh:
- Test overwriting the main shell trap in a subshell for all
  pseudosignals.

Makes progress on: https://github.com/ksh93/ksh/issues/155

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-23:
+
+- Fixed: when the DEBUG trap was redefined in a subshell, the DEBUG trap in
+  the parent environment was corrupted or the shell crashed.
+
 2021-01-22:
 
 - Compile-time shell options can now be edited in src/cmd/ksh93/SHOPT.sh