Fix UnboundLocalError when reading byte_stream

Author: ekawas-vrify

src/shapefile.py

@@ -1493,6 +1493,7 @@ class _HasM(_CanHaveBBox):
     def _read_ms_from_byte_stream(
         b_io: ReadSeekableBinStream, nPoints: int, next_shape: int
     ) -> tuple[MBox, list[float | None]]:
+        mbox = None  # Ensure mbox is always defined
         if next_shape - b_io.tell() >= 16:
             mbox = unpack("<2d", b_io.read(16))
         # Measure values less than -10e38 are nodata values according to the spec