Fix SourceMap::start_point for empty spans

When the span is empty, it doesn't really have a first character. Even
worse, when it's empty at the end of the file, adding a byte offset will
make it be out of bounds. So we just return the empty span in these
cases.

compiler/rustc_span/src/source_map.rs

@@ -881,7 +881,12 @@ impl SourceMap {
     }
 
     /// Returns a new span representing just the first character of the given span.
+    /// When the span is empty, the same empty span is returned.
     pub fn start_point(&self, sp: Span) -> Span {
+        if sp.is_empty() {
+            return sp;
+        }
+
         let width = {
             let sp = sp.data();
             let local_begin = self.lookup_byte_offset(sp.lo);