sh: Fix ""$@, which should not use the special case for "$@".

"$@" should expand to no words if there are no positional parameters, but
""$@ should always expand to at least an empty word.

bin/sh/expand.c

@@ -249,7 +249,8 @@ argstr(char *p, int flag)
 		case CTLQUOTEMARK:
 			lit_quoted = 1;
 			/* "$@" syntax adherence hack */
-			if (p[0] == CTLVAR && p[2] == '@' && p[3] == '=')
+			if (p[0] == CTLVAR && (p[1] & VSQUOTE) != 0 &&
+			    p[2] == '@' && p[3] == '=')
 				break;
 			if ((flag & EXP_FULL) != 0)
 				USTPUTC(c, expdest);

bin/sh/tests/parameters/positional8.0

@@ -0,0 +1,31 @@
+# $FreeBSD$
+
+failures=''
+ok=''
+
+testcase() {
+	code="$1"
+	expected="$2"
+	oIFS="$IFS"
+	eval "$code"
+	IFS='|'
+	result="$#|$*"
+	IFS="$oIFS"
+	if [ "x$result" = "x$expected" ]; then
+		ok=x$ok
+	else
+		failures=x$failures
+		echo "For $code, expected $expected actual $result"
+	fi
+}
+
+testcase 'shift $#; set -- ""$*'		'1|'
+testcase 'shift $#; set -- $*""'		'1|'
+testcase 'shift $#; set -- ""$@'		'1|'
+testcase 'shift $#; set -- $@""'		'1|'
+testcase 'shift $#; set -- """$*"'		'1|'
+testcase 'shift $#; set -- "$*"""'		'1|'
+testcase 'shift $#; set -- """$@"'		'1|'
+testcase 'shift $#; set -- "$@"""'		'1|'
+
+test "x$failures" = x