Fix code folding when folding ends on the last line

Previously, a final FoldBlock that begins one line after the last
block and ends at the last line was always created. If the last block
ended on the last line of the listing, this resulted in an
ArrayIndexOutOfBoundException.

This commit updates the converter to only create the final FoldBlock if
it's starting line is on or before the last line of the listing.

Fixes gh-26

Author: wilkinsona

src/main/java/io/spring/asciidoctor/backend/codetools/FoldListingContentConverter.java

@@ -271,7 +271,9 @@ static FoldBlocks get(String[] lines, Folders folders) {
 					}
 				}
 			}
-			foldBlocks.add(createFoldArea(folder, lines, start, lines.length - 1));
+			if (start < lines.length) {
+				foldBlocks.add(createFoldArea(folder, lines, start, lines.length - 1));
+			}
 			return new FoldBlocks(foldBlocks.stream().filter(FoldBlock::hasLines));
 		}
 

src/test/java/io/spring/asciidoctor/backend/codetools/FoldListingContentConverterTests.java

@@ -84,4 +84,9 @@ void convertWhenHasCodeFoldingDisabledAsBlockAttributeReturnsUnfolded(ConvertedH
 		assertThat(html.getElementHtml("code")).satisfies(expected);
 	}
 
+	@Test
+	void convertWhenFoldingIsTurnedOffOnTheLastLineReturnsFolded(ConvertedHtml html, ExpectedHtml expected) {
+		assertThat(html.getElementHtml("code")).satisfies(expected);
+	}
+
 }

src/test/resources/io/spring/asciidoctor/backend/codetools/FoldListingContentConverterTests_convertWhenFoldingIsTurnedOffOnTheLastLineReturnsFolded.adoc

@@ -0,0 +1,10 @@
+= Test Document
+
+[source,java]
+----
+// @fold:on
+public class Test {
+
+}
+// @fold:off
+----

src/test/resources/io/spring/asciidoctor/backend/codetools/FoldListingContentConverterTests_convertWhenFoldingIsTurnedOffOnTheLastLineReturnsFolded_expected.html

@@ -0,0 +1,4 @@
+<span class="fold-block hide-when-folded">public class Test {
+
+}
+</span>