Several functions accept a buffer size but never actually check whether
the buffer overflows during reading/writing. This fixes all cases where
a size variable has explicitly been marked as unused (through a (void)
cast).
This was discovered while investigating an assertion caused by
7cc691f332f26802c64cdc47e17bff8b_signal_sigabrt_7ffff6d59425_2247_2509.pdf
among others.
Thanks to Mateusz Jurczyk and Gynvael Coldwind of the Google Security
Team for providing the example files.
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