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:
https://src.fedoraproject.org/rpms/ksh/blob/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.

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-26"
+#define SH_RELEASE	"93u+m 2020-09-27"

src/cmd/ksh93/sh/lex.c

@@ -1541,6 +1541,7 @@ int sh_lex(Lex_t* lp)
 
 /*
  * read to end of command substitution
+ * of the form $(...)
  */
 static int comsub(register Lex_t *lp, int endtok)
 {
@@ -1593,7 +1594,14 @@ static int comsub(register Lex_t *lp, int endtok)
 				if(n==4)
 					break;
 				if(sh_lexstates[ST_NAME][c])
+				{
+					if(c==' ' || c=='\t')
+					{
+						n = 0;
+						continue;
+					}
 					goto skip;
+				}
 				word[n++] = c;
 			}
 			if(sh_lexstates[ST_NAME][c]==S_BREAK)

src/cmd/ksh93/tests/basic.sh

@@ -716,5 +716,14 @@ else	err_exit "warning: skipping argv rewrite test due to noncompliant 'ps' util
 	let Errors--
 fi
 
+# ======
+# https://bugzilla.redhat.com/1241013
+got=$(eval 'x=$(for i in test; do case $i in test) true;; esac; done)' 2>&1) \
+|| err_exit "case in a for loop inside a \$(comsub) caused syntax error (got $(printf %q "$got"))"
+got=$(eval 'x=${ for i in test; do case $i in test) true;; esac; done; }' 2>&1) \
+|| err_exit "case in a for loop inside a \${ comsub; } caused syntax error (got $(printf %q "$got"))"
+got=$(eval 'x=`for i in test; do case $i in test) true;; esac; done`' 2>&1) \
+|| err_exit "case in a for loop inside a \`comsub\` caused syntax error (got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))