Fix issue where we can't find previous sibling because of nested slots

src/compiler/compile/css/Selector.ts

@@ -483,11 +483,11 @@ function find_previous_sibling_ignoring_slots(node: INode): INode {
 		}
 
 		// @ts-ignore
-		if (!current_node.prev && current_node.parent.type === 'Slot') {
-			current_node = current_node.parent.prev;
-		} else {
-			current_node = current_node.prev;
+		while (!current_node.prev && current_node.parent && current_node.parent.type === 'Slot') {
+			current_node = current_node.parent;
 		}
+		current_node = current_node.prev;
+
 	// @ts-ignore
 	} while (current_node && current_node.type === 'Slot');
 

test/css/samples/general-siblings-combinator-nested-slots-flattened/expected.css

@@ -0,0 +1 @@
+h1.svelte-xyz~p.svelte-xyz{color:red}

test/css/samples/general-siblings-combinator-nested-slots-flattened/input.svelte

@@ -0,0 +1,7 @@
+<slot><slot><h1>Heading 1</h1></slot></slot><span>Span 1</span><span>Span 2</span><slot><slot><p>Paragraph 2</p></slot></slot>
+
+<style>
+  h1 ~ p {
+    color: red;
+  }
+</style>

test/css/samples/siblings-combinator-nested-slots-flattened/expected.css

@@ -0,0 +1 @@
+h1.svelte-xyz+span.svelte-xyz{color:red}

test/css/samples/siblings-combinator-nested-slots-flattened/input.svelte

@@ -0,0 +1,7 @@
+<slot><slot><slot><h1>test</h1></slot></slot></slot><slot><slot><span>Hello</span></slot></slot>
+
+<style>
+  h1 + span {
+    color: red;
+  }
+</style>