fix memory leak in mpi_miller_rabin()

Fixes memory leak in mpi_miller_rabin() that occurs when the function has
failed to obtain a usable random 'A' 30 turns in a row.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

library/bignum.c

@@ -2329,7 +2329,8 @@ static int mpi_miller_rabin( const mbedtls_mpi *X, size_t rounds,
             }
 
             if (count++ > 30) {
-                return MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
+                ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
+                goto cleanup;
             }
 
         } while ( mbedtls_mpi_cmp_mpi( &A, &W ) >= 0 ||