Fix lexing of 'case' in do...done in a $(comsub) (rhbz#1241013)

The following caused a spurious syntax error:

$ x=$(for i in 1; do case $i in word) true;; esac; done)
-ksh: syntax error: `;;' unexpected

Prior discussion:
https://bugzilla.redhat.com/1241013

Original patch, backported from 93v- beta, applied without change:
642af4d6/f/ksh-20120801-parserfix.patch

NEWS

@@ -3,6 +3,15 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-27:
+
+- The shell's lexical analisys of a 'case' statement within a do...done block
+  within a command substitution of the form $(...) has been fixed so that code
+  like the following no longer throws a spurious syntax error:
+	x=$(for i in 1; do case $i in word) true;; esac; done)
+  Previously, this required a leading parenthesis before 'word', although the
+  syntax error claimed that the ';;' was unexpected.
+
 2020-09-26:
 
 - 'whence -f' now completely ignores the existence of functions, as documented.