LibCrypto: Avoid overly big allocs in intermediate ModularPower results

If we don't limit the sizes of the intermediate results, they will grow
indefinitely, causing each iteration to take longer and longer (in both
memcpy time, and algorithm runtime).
While calculating the trimmed length is fairly expensive, it's a small
cost to pay for uniform iteration times.

Userland/Libraries/LibCrypto/NumberTheory/ModularFunctions.cpp

@@ -150,6 +150,13 @@ UnsignedBigInteger ModularPower(const UnsignedBigInteger& b, const UnsignedBigIn
         UnsignedBigInteger::multiply_without_allocation(base, base, temp_1, temp_2, temp_3, temp_4, temp_multiply);
         UnsignedBigInteger::divide_without_allocation(temp_multiply, m, temp_1, temp_2, temp_3, temp_4, temp_quotient, temp_remainder);
         base.set_to(temp_remainder);
+
+        // Note that not clamping here would cause future calculations (multiply, specifically) to allocate even more unused space
+        // which would then persist through the temp bigints, and significantly slow down later loops.
+        // To avoid that, we can clamp to a specific max size, or just clamp to the min needed amount of space.
+        ep.clamp_to_trimmed_length();
+        exp.clamp_to_trimmed_length();
+        base.clamp_to_trimmed_length();
     }
     return exp;
 }