Skip redundant checks for NULL f_rng

Unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined, it's no longer possible for
f_rng to be NULL at the places that randomize coordinates.

Eliminate the NULL check in this case:
- it makes it clearer to reviewers that randomization always happens (unless
  the user opted out at compile time)
- a NULL check in a place where it's easy to prove the value is never NULL
  might upset or confuse static analyzers (including humans)
- removing the check saves a bit of code size

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/ecp.c

@@ -1948,7 +1948,9 @@ static int ecp_mul_comb_core( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R
         i = d;
         MBEDTLS_MPI_CHK( ecp_select_comb( grp, R, T, T_size, x[i] ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &R->Z, 1 ) );
+#if defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
         if( f_rng != 0 )
+#endif
             MBEDTLS_MPI_CHK( ecp_randomize_jac( grp, R, f_rng, p_rng ) );
     }
 
@@ -2081,7 +2083,9 @@ final_norm:
      *
      * Avoid the leak by randomizing coordinates before we normalize them.
      */
+#if defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
     if( f_rng != 0 )
+#endif
         MBEDTLS_MPI_CHK( ecp_randomize_jac( grp, RR, f_rng, p_rng ) );
 
     MBEDTLS_ECP_BUDGET( MBEDTLS_ECP_OPS_INV );
@@ -2484,7 +2488,9 @@ static int ecp_mul_mxz( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
     MOD_ADD( RP.X );
 
     /* Randomize coordinates of the starting point */
+#if defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
     if( f_rng != NULL )
+#endif
         MBEDTLS_MPI_CHK( ecp_randomize_mxz( grp, &RP, f_rng, p_rng ) );
 
     /* Loop invariant: R = result so far, RP = R + P */
@@ -2517,7 +2523,9 @@ static int ecp_mul_mxz( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
      *
      * Avoid the leak by randomizing coordinates before we normalize them.
      */
+#if defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
     if( f_rng != NULL )
+#endif
         MBEDTLS_MPI_CHK( ecp_randomize_mxz( grp, R, f_rng, p_rng ) );
 
     MBEDTLS_MPI_CHK( ecp_normalize_mxz( grp, R ) );