Fix code scanning alert no. 470: Multiplication result converted to larger type

[cooljeanius]

Fixes https://github.com/cooljeanius/apple-gdb-1824/security/code-scanning/470

To fix the problem, we need to ensure that the multiplication is performed using a larger integer type to prevent overflow. We can cast one of the operands to size_t before performing the multiplication. This will ensure that the multiplication is done using the larger type, thus avoiding overflow.

The specific changes required are:

1. Cast max_y to size_t before multiplying it with max_x in the memset function calls.
2. Similarly, cast max_y to size_t before multiplying it with max_x in the gdb_malloc function calls.

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[cooljeanius]

just some simple casts; ok