Work around false-positive warning.

The warning was:

```
s_mp_exptmod_fast.c: In function ‘s_mp_exptmod_fast’:
s_mp_exptmod_fast.c:188:21: warning: ‘mp’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  188 |          if ((err = redux(&res, P, mp)) != MP_OKAY)               goto LBL_RES;
      |                     ^
s_mp_exptmod_fast.c:25:18: note: ‘mp’ was declared here
   25 |    mp_digit buf, mp;
      |                  ^
```

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

Author: sjaeckel

s_mp_exptmod_fast.c

@@ -22,7 +22,7 @@
 mp_err s_mp_exptmod_fast(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y, int redmode)
 {
    mp_int  M[TAB_SIZE], res;
-   mp_digit buf, mp;
+   mp_digit buf, mp = 0;
    int     bitbuf, bitcpy, bitcnt, mode, digidx, x, y, winsize;
    mp_err   err;