Fix static initializer warning

In a hypothetical build with no curves, or in the future when we add a
new curve type and possibly forget updating this function with a new
block for the new type, we write to `ret` at the beginning or the
function then immediately overwrite it with MPI_CHK(check_privkey),
which static analyzers understandably find questionable.

Use `ret` here and check the key only if it was actually set.

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

library/ecp.c

@@ -3288,7 +3288,10 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
         MBEDTLS_MPI_CHK(mbedtls_mpi_read_binary(&key->d, buf, buflen));
     }
 #endif
+
+    if (ret == 0) {
         MBEDTLS_MPI_CHK(mbedtls_ecp_check_privkey(&key->grp, &key->d));
+    }
 
 cleanup: