parser: Fix backquote support in here-document EOF mark

Currently using backquotes in a here-document EOF mark is broken
because dash tries to do command substitution on it.  This patch
fixes it by checking whether we're looking for an EOF mark during
tokenisation.

Reported-by: Harald van Dijk <harald@gigawatt.nl>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/parser.c

@@ -1037,6 +1037,11 @@ toggledq:
 				}
 				break;
 			case CBQUOTE:	/* '`' */
+				if (checkkwd & CHKEOFMARK) {
+					USTPUTC('`', out);
+					break;
+				}
+
 				PARSEBACKQOLD();
 				break;
 			case CEOF: