bigint.c: increase karatsuba scratch allocation safety margin

the previous fixed safety margin of 8 limbs was insufficient for certain
edge cases involving deep recursion levels in karatsuba multiplication,
as discovered by oss-fuzz. changed to proportional margin (~12.5% plus
fixed overhead of 16) that scales with input size.

this prevents potential buffer overruns in deeply nested karatsuba
multiplications while maintaining efficiency for typical cases.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bigint/core/bigint.c

@@ -1112,8 +1112,11 @@ mpz_mul(mpz_ctx_t *ctx, mpz_t *ww, mpz_t *u, mpz_t *v)
   mpz_realloc(ctx, ww, result_size);
 
   size_t scratch_size = karatsuba_scratch_size(u->sz, v->sz);
-  /* Add safety margin to account for rounding in recursive partitioning */
-  scratch_size += 8;
+  /* Add safety margin proportional to scratch size.
+   * While the calculation is mathematically exact, empirical testing
+   * (valgrind, oss-fuzz) reveals edge cases requiring extra space.
+   * Proportional margin scales better than fixed for large inputs. */
+  scratch_size += (scratch_size >> 3) + 16;
   size_t pool_state = pool_save(ctx);
   mp_limb *scratch = NULL;