fix: Do not call unref on a borrowed pointer

Tau Gärtlirequested to merge
tautropfli/pygobject:fix-double-unref into main

gi_base_info_get_container returns a borrowed pointer, so we should not call gi_base_info_unref on it.

This caused a segfault when repeatedly accessing an array field with length annotation on a struct.

Fixes #738 (closed)

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