Garbage output when using interpolation with unquoted string

[vaadin-bot]

Originally by @Artur-

---

$pathFix: "../../../valoexamples";
$fontfile: "fonts/Vaadin-Icons";
$full: #{$pathFix}/#{$fontfile};

.foo {
  full: $full;
}
@include v-font(VaadinIcons, $full);

Should, when used with Valo, output

.foo {
	full: ../../../valoexamples/fonts/Vaadin-Icons;
}

@font-face {
	font-family: VaadinIcons;
	font-weight: normal;
	font-style: normal;
	src: url(../valo/util/bourbon/css3/../../../valoexamples/fonts/Vaadin-Icons.eot);
...

but instead it outputs

.foo {
	full: ../../../valoexamples/fonts/Vaadin-Icons;
}

@font-face {
	font-family: VaadinIcons;
	font-weight: normal;
	font-style: normal;
	src: url(../valo/util/bourbon/css3/�.eot);
...

No warnings or errors are output. Quoting $full makes it works as expected.

---

Imported from https://dev.vaadin.com/ issue #18910