FormattedExcinfo.get_source: avoid crash when line number is out-of-bounds/negative

pytest could crash given pathological AST position attributes, which shouldn't happen when testing real Python code, but could happen when testing AST produced by e.g. Hylang.

Another example of the failure is in the nightly CI for the JAX project: https://github.com/google/jax/actions/runs/4607513902/jobs/8142126075

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>
Co-authored-by: Jake VanderPlas <jakevdp@google.com>

src/_pytest/_code/code.py

@@ -743,11 +743,13 @@ class FormattedExcinfo:
     ) -> List[str]:
         """Return formatted and marked up source lines."""
         lines = []
-        if source is None or line_index >= len(source.lines):
+        if source is not None and line_index < 0:
+            line_index += len(source)
+        if source is None or line_index >= len(source.lines) or line_index < 0:
+            # `line_index` could still be outside `range(len(source.lines))` if
+            # we're processing AST with pathological position attributes.
             source = Source("???")
             line_index = 0
-        if line_index < 0:
-            line_index += len(source)
         space_prefix = "    "
         if short:
             lines.append(space_prefix + source.lines[line_index].strip())