Fix coverity issues in EC after #11807

Fix coverity issues in EC after #11807

This should fix 2 issues detected by Coverity and introduced with
https://github.com/openssl/openssl/pull/11807

- CID 1463577:  Memory - corruptions  (ARRAY_VS_SINGLETON)
- CID 1463573:  Memory - corruptions  (ARRAY_VS_SINGLETON)

In practice the tests seem to show that they both aren't real issues,
yet I believe this small change should appease the scanner and at the
same time improve clarity for the reader.

Here is the original report:

```
** CID 1463577:  Memory - corruptions  (ARRAY_VS_SINGLETON)

________________________________________________________________________________________________________
*** CID 1463577:  Memory - corruptions  (ARRAY_VS_SINGLETON)
/crypto/ec/ec_lib.c: 1123 in EC_POINT_mul()
1117
1118         if (group->meth->mul != NULL)
1119             ret = group->meth->mul(group, r, g_scalar, point != NULL
1120                                    && p_scalar != NULL, &point, &p_scalar, ctx);
1121         else
1122             /* use default */
   CID 1463577:  Memory - corruptions  (ARRAY_VS_SINGLETON)
   Passing "&point" to function "ec_wNAF_mul" which uses it as an array. This might corrupt or misinterpret adjacent memory locations.
1123             ret = ec_wNAF_mul(group, r, g_scalar, point != NULL
1124                               && p_scalar != NULL, &point, &p_scalar, ctx);
1125
1126     #ifndef FIPS_MODULE
1127         BN_CTX_free(new_ctx);
1128     #endif

** CID 1463573:  Memory - corruptions  (ARRAY_VS_SINGLETON)

________________________________________________________________________________________________________
*** CID 1463573:  Memory - corruptions  (ARRAY_VS_SINGLETON)
/crypto/ec/ec_lib.c: 1123 in EC_POINT_mul()
1117
1118         if (group->meth->mul != NULL)
1119             ret = group->meth->mul(group, r, g_scalar, point != NULL
1120                                    && p_scalar != NULL, &point, &p_scalar, ctx);
1121         else
1122             /* use default */
   CID 1463573:  Memory - corruptions  (ARRAY_VS_SINGLETON)
   Passing "&p_scalar" to function "ec_wNAF_mul" which uses it as an array. This might corrupt or misinterpret adjacent memory locations.
1123             ret = ec_wNAF_mul(group, r, g_scalar, point != NULL
1124                               && p_scalar != NULL, &point, &p_scalar, ctx);
1125
1126     #ifndef FIPS_MODULE
1127         BN_CTX_free(new_ctx);
1128     #endif
```

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11919)