FormattedExcinfo.get_source: Avoid a crash when source.lines == 0

src/_pytest/_code/code.py

@@ -743,11 +743,11 @@ def get_source(
     ) -> List[str]:
         """Return formatted and marked up source lines."""
         lines = []
-        if source is None or line_index >= len(source.lines):
-            source = Source("???")
-            line_index = 0
         if line_index < 0:
             line_index += len(source)
+        if source is None or line_index >= len(source.lines) or line_index < 0:
+            source = Source("???")
+            line_index = 0
         space_prefix = "    "
         if short:
             lines.append(space_prefix + source.lines[line_index].strip())