Parse empty docblock lines for backtraces without panicking

Fixes #263.

binrw/tests/error.rs

@@ -293,6 +293,18 @@ fn no_seek_unit_enum() {
     }
 }
 
+#[test]
+fn parse_backtrace_with_empty_comment_lines() {
+    #[derive(binrw::BinRead)]
+    pub struct Test {
+        /// Blank next line has no whitespace…
+
+        /// …but it is part of the same span, and needs to not crash the
+        /// backtrace formatter
+        _a: u32,
+    }
+}
+
 #[test]
 fn show_backtrace() {
     use alloc::borrow::Cow;

binrw_derive/src/binrw/backtrace/mod.rs

@@ -52,7 +52,7 @@ impl BacktraceFrame {
                                 format!("{}{}", " ".repeat(spaces_to_add), line)
                             }
                         } else {
-                            line[min_whitespace..].to_owned()
+                            line.get(min_whitespace..).unwrap_or_default().to_owned()
                         };
 
                         (min_whitespace + 1, line)