Don't panic on empty file/buffer (#108)

helix-core/src/match_brackets.rs

@@ -25,6 +25,10 @@ pub fn find(syntax: &Syntax, doc: &Rope, pos: usize) -> Option<usize> {
     }
 
     let start_byte = node.start_byte();
+    let len = doc.len_bytes();
+    if start_byte >= len {
+        return None;
+    }
     let end_byte = node.end_byte() - 1; // it's end exclusive
     let start_char = doc.byte_to_char(start_byte);
     let end_char = doc.byte_to_char(end_byte);