Fix crash when rendering a code block without a closing fence (#112)

Author: Guille Gonzalez

Sources/MarkdownUI/Rendering/AttributedStringRenderer.swift

@@ -266,8 +266,10 @@ extension AttributedStringRenderer {
     state.addFirstLineIndent()
 
     var code = codeBlock.code.replacingOccurrences(of: "\n", with: String.lineSeparator)
-    // Remove the last line separator
-    code.removeLast()
+    if !code.isEmpty {
+      // Remove the last line separator
+      code.removeLast()
+    }
 
     return renderParagraph(.init(text: [.text(code)]), hasSuccessor: hasSuccessor, state: state)
   }

Tests/MarkdownUITests/MarkdownTests.swift

@@ -239,6 +239,20 @@
       assertSnapshot(matching: view, as: .image(layout: layout), named: platformName)
     }
 
+    func testOpenCodeBlock() {
+      let view = Markdown(
+        #"""
+        An code block without a closing fence:
+
+        ```swift
+        """#
+      )
+      .background(Color.orange)
+      .padding()
+
+      assertSnapshot(matching: view, as: .image(layout: layout), named: platformName)
+    }
+
     func testVerbatimHTML() {
       let view = Markdown(
         #"""