gh-153578: Fix out-of-bounds write in bytearray.extend() with a reentrant __buffer__ (GH-153579)

bytearray.extend() clamped only the high bound of the append range to the
current size after acquiring the argument's buffer, so a __buffer__ that shrinks
the bytearray left the low bound past the high bound and ran a negative-size
memmove. Clamp the low bound too, matching bytearray.__iadd__.