libtommath: Fix possible integer overflow CVE-2023-36328

Cherry picked from libtommath 7bbc1f8e4fe6dce75055957645117180768efb15.

Vulnerability Detail:
  CVE Identifier: CVE-2023-36328
  Description: Integer Overflow vulnerability in mp_grow in libtom
    libtommath before commit beba892bc0d4e4ded4d667ab1d2a94f4d75109a9,
    allows attackers to execute arbitrary code and cause a denial of
    service (DoS).
  Reference: https://nvd.nist.gov/vuln/detail/CVE-2023-36328

Reported-by: https://github.com/Crispy-fried-chicken

lib/hcrypto/libtommath/bn_mp_mul_2d.c

@@ -9,6 +9,10 @@ mp_err mp_mul_2d(const mp_int *a, int b, mp_int *c)
    mp_digit d;
    mp_err   err;
 
+   if (b < 0) {
+      return MP_VAL;
+   }
+
    /* copy */
    if (a != c) {
       if ((err = mp_copy(a, c)) != MP_OKAY) {