Move MPI initialization to start of function

This prevents a call to mbedtls_mpi_free() on uninitialized data when
USE_PSA_INIT() fails.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

tests/suites/test_suite_x509write.function

@@ -435,10 +435,11 @@ void x509_set_serial_check()
     mbedtls_mpi serial_mpi;
     uint8_t invalid_serial[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN + 1];
 
+    mbedtls_mpi_init(&serial_mpi);
+
     USE_PSA_INIT();
     memset(invalid_serial, 0x01, sizeof(invalid_serial));
 
-    mbedtls_mpi_init(&serial_mpi);
     TEST_EQUAL(mbedtls_mpi_read_binary(&serial_mpi, invalid_serial,
                                        sizeof(invalid_serial)), 0);
     TEST_EQUAL(mbedtls_x509write_crt_set_serial(&ctx, &serial_mpi),