gettext.GNUTranslations raises struct.error instead of OSError on a malformed .mo file

Bug report

Bug description:

gettext.GNUTranslations parses a .mo file by unpacking its header and seek
tables with struct.unpack, but never checks the file is long enough. A
truncated file (shorter than the 20-byte header) or one whose seek tables point
past the end raises a raw struct.error instead of the OSError the module
already raises for other malformed input (a bad magic number, a bad version, or
an out-of-range string offset). The documentation states OSError is raised for
problems while reading the file, so the struct.error is an undocumented leak.

import gettext, io
gettext.GNUTranslations(io.BytesIO(b"\xde\x12\x04\x95")) # raises struct.error

CPython versions tested on:

3.13, 3.14, 3.15, 3.16

Operating systems tested on:

macOS

Linked PRs

• gh-153606

Comienza en gettext.GNUTranslations y reproduce con io.BytesIO los casos de archivos .mo truncados y fuera de rango descritos. El trabajo estará terminado cuando las entradas malformadas generen OSError de forma coherente en lugar de dejar escapar struct.error, con cobertura de regresión para los casos notificados.