gh-127681: Fix overflow vulnerabilities in PyMem_Malloc calls #3396

Job	Run time
x86_64-apple-darwin/clang	20m 16s
aarch64-unknown-linux-gnu/gcc	6m 36s
x86_64-unknown-linux-gnu/gcc	10m 21s
x86_64-unknown-linux-gnu/gcc-free-threading	7m 58s
aarch64-apple-darwin/clang	10m 15s
	55m 26s

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