mruby-bigint: maintain canonical sn=0 when trim reduces sz to 0

mpz_t treats sn (sign) as the canonical "is zero" flag (zero_p(x) :=
(x)->sn == 0). When an arithmetic operation produces a value whose
limbs trim to zero size, sn must be reset to 0 to preserve the
invariant. Several call sites already enforced this locally (e.g.
mpz_sub line 616); make trim() responsible so every caller benefits.

Without this, an inconsistent zero bignum (sn!=0, sz=0) can flow into
mpz_sqr, miss the zero_p guard, and reach mpz_init_heap with hint=0
where mpn_zero(NULL, 0) invokes UB (memset() declares its first
argument nonnull). The trip survives at runtime on glibc but is
formally undefined behavior, flagged by UBSan via Integer#pow(b,e,m)
with specific operands.

close #6849

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

mrbgems/mruby-bigint/core/bigint.c

@@ -373,6 +373,8 @@ trim(mpz_t *x)
   while (x->sz && x->p[x->sz-1] == 0) {
     x->sz--;
   }
+  /* Maintain invariant: sz == 0 implies sn == 0 (zero is canonical). */
+  if (x->sz == 0) x->sn = 0;
 }
 
 /* z = x + y, without regard for sign */