Coverity 1528485: Remove unused assignment of wvalue

wvalue is always initialized at the beginning of each cycle
and used only within the cycle

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from #21145)

crypto/bn/bn_exp.c

@@ -304,7 +304,7 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
 int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                     const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
 {
-    int i, j, bits, ret = 0, wstart, wend, window, wvalue;
+    int i, j, bits, ret = 0, wstart, wend, window;
     int start = 1;
     BIGNUM *d, *r;
     const BIGNUM *aa;
@@ -384,7 +384,6 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     start = 1;                  /* This is used to avoid multiplication etc
                                  * when there is only the value '1' in the
                                  * buffer. */
-    wvalue = 0;                 /* The 'value' of the window */
     wstart = bits - 1;          /* The top bit of the window */
     wend = 0;                   /* The bottom bit of the window */
 
@@ -404,6 +403,8 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     if (!bn_to_mont_fixed_top(r, BN_value_one(), mont, ctx))
         goto err;
     for (;;) {
+        int wvalue;             /* The 'value' of the window */
+
         if (BN_is_bit_set(p, wstart) == 0) {
             if (!start) {
                 if (!bn_mul_mont_fixed_top(r, r, r, mont, ctx))
@@ -446,7 +447,6 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 
         /* move the 'window' down further */
         wstart -= wend + 1;
-        wvalue = 0;
         start = 0;
         if (wstart < 0)
             break;