Fix crash when converting malformed HTML with unmatched closing tag

Sources/HTMLStreamer/AttributedStringConverter.swift

@@ -75,10 +75,12 @@ public struct AttributedStringConverter<Callbacks: HTMLConversionCallbacks> {
             case .endTag(let name):
                 handleEndTag(name)
                 // if we have a non-default action for the current element, the run finishes here
-                if actionStack.last != .default {
+                if let action = actionStack.last {
+                    if action != .default {
                         finishRun()
                     }
                     actionStack.removeLast()
+                }
             case .doctype:
                 // ignored
                 continue

Sources/HTMLStreamer/TextConverter.swift

@@ -55,10 +55,12 @@ public struct TextConverter<Callbacks: HTMLConversionCallbacks> {
                 handleStartTag(name, selfClosing: selfClosing, attributes: attributes)
             case .endTag(let name):
                 handleEndTag(name)
-                if actionStack.last != .default {
+                if let action = actionStack.last {
+                    if action != .default {
                         finishRun()
                     }
                     actionStack.removeLast()
+                }
             case .comment, .doctype:
                 break
             }

Tests/HTMLStreamerTests/AttributedStringConverterTests.swift

@@ -352,4 +352,8 @@ final class AttributedStringConverterTests: XCTestCase {
         XCTAssertEqual(convert(#"<a href="https://example.com"><span class="invisible">https://</span><span>example.com</span><span class="invisible"></span></a>"#, callbacks: Callbacks.self), result)
     }
     
+    func testMalformedOnlyClosingTag() {
+        XCTAssertEqual(convert("</span>"), .init())
+    }
+    
 }

Tests/HTMLStreamerTests/TextConverterTests.swift

@@ -83,4 +83,8 @@ final class TextConverterTests: XCTestCase {
         XCTAssertEqual(convert(#"<a href="https://example.com"><span class="invisible">https://</span><span>example.com</span><span class="invisible"></span></a>"#, callbacks: Callbacks.self), "example.com")
     }
     
+    func testMalformedOnlyClosingTag() {
+        XCTAssertEqual(convert("</span>"), "")
+    }
+    
 }