refactor: remove unnecessary malloc result casts

[theStack]

While working on benchmark code for #1765, I noticed that in some instances we explicitly cast malloc results in the codebase. It seems that there is no good reason to do this in C, and it's even considered bad practice, see e.g. https://stackoverflow.com/a/605858.

This commit touches mostly test code, the only two functions used in production are secp256k1_context_{create,clone}. Instances were found manually via $ git grep "malloc(".

[real-or-random]

Weak Concept ACK && Code Review ACK 97b3c47

I think that's mostly a matter of style. But yeah, this results in less code without loss of readability, and it's a bit more idiomatic, and that's why I agree that the new style is better.

IIUC the casts are necessary in C++, and this may be the primary reason why we these casts (e.g., I assume Pieter is more fluent in C++ than in C).

[w0xlt]

ACK 97b3c47