This fixes erroneous syntax errors in parameter expansions such as

${var:-wor)d} or ${var+w(ord}. The parentheses now correctly lose
their normal grammatical meaning within the braces. Fix by Eric
Scrivner (@etscrivner) from July 2018 backported from ksh2020.

This fix complies with POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

src/cmd/ksh93/sh/lex.c: sh_lex():
- Set the ST_QUOTE state when analysing a modifier with parameter
  expansions using operators ':', '-', '+', '='. This state causes
  subsequent characters (including parentheses) to be considered
  quoted, suppressing their normal grammatical meaning.

src/cmd/ksh93/sh/macro.c: varsub():
- Same for skipping the expansion.

Fixes: https://github.com/ksh93/ksh/issues/126
Prior discussion: https://github.com/att/ast/issues/475

src/cmd/ksh93/sh/macro.c

@@ -1628,7 +1628,8 @@ retry1:
 			}
 			else
 			{
-				sh_lexskip(lp,RBRACE,0,(!newops&&mp->quote)?ST_QUOTE:ST_NESTED);
+				int state = (!newops && mp->quote || c=='-' || c=='+' || c=='=') ? ST_QUOTE : ST_NESTED;
+				sh_lexskip(lp, RBRACE, 0, state);
 				stkseek(stkp,offset);
 			}
 			argp=stkptr(stkp,offset);