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

從 gettext.GNUTranslations 開始，使用 io.BytesIO 重現所描述的截斷 .mo 與超出範圍的 .mo 情況。當格式錯誤的輸入一致地引發 OSError，而不是洩漏 struct.error，並且為回報的情況補上回歸涵蓋時，這項工作就完成了。