fix two possible buffer overflow bugs

[Frank-Buss]

Note

Adds strict input-size validation during session initialization to prevent buffer overflows.

• In InitSession, validate disc_int_size against sizeof(disc) and throw on invalid values
• Validate form_size against sizeof(initial_form_s) and throw on invalid values

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[Frank-Buss]

PS: technically the test for <= 0 is a semantic change, but disc size can't be 0 because the discriminant is a prime used to define the class group, which can't be represented with zero bytes. Additionally, atoi returns 0 for non-numeric input, so this catches malformed sizes. And the binary quadratic form also requires actual coefficients, so can't be 0 either. Finally, form_size is a signed char, so <= 0 catches wire values 128-255 that wrap negative.