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

src/cmd/ksh93/sh/lex.c

@@ -1541,6 +1541,7 @@ breakloop:
 
 /*
  * 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)