fix crashes on prev_boundary

src/grapheme.rs

@@ -629,6 +629,9 @@ impl GraphemeCursor {
         if self.offset == 0 {
             return Ok(None);
         }
+        if self.offset == chunk_start {
+            return Err(GraphemeIncomplete::PrevChunk);
+        }
         let mut iter = chunk[..self.offset - chunk_start].chars().rev();
         let mut ch = iter.next().unwrap();
         loop {
@@ -652,6 +655,7 @@ impl GraphemeCursor {
                     self.decide(true);
                 } else {
                     self.resuming = true;
+                    self.cat_after = Some(gr::grapheme_category(ch));
                     return Err(GraphemeIncomplete::PrevChunk);
                 }
             }
@@ -682,3 +686,19 @@ fn test_grapheme_cursor_chunk_start_require_precontext() {
     c.provide_context(&s[..1], 0);
     assert_eq!(c.is_boundary(&s[1..], 1), Ok(false));
 }
+
+#[test]
+fn test_grapheme_cursor_prev_boundary() {
+    let s = "abcd";
+    let mut c = GraphemeCursor::new(3, s.len(), true);
+    assert_eq!(c.prev_boundary(&s[2..], 2), Err(GraphemeIncomplete::PrevChunk));
+    assert_eq!(c.prev_boundary(&s[..2], 0), Ok(Some(2)));
+}
+
+#[test]
+fn test_grapheme_cursor_prev_boundary_chunk_start() {
+    let s = "abcd";
+    let mut c = GraphemeCursor::new(2, s.len(), true);
+    assert_eq!(c.prev_boundary(&s[2..], 2), Err(GraphemeIncomplete::PrevChunk));
+    assert_eq!(c.prev_boundary(&s[..2], 0), Ok(Some(1)));
+}