Restrict input parameter size for ecp_mod_p521_raw

The imput mpi parameter must have twice as many limbs as the modulus.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index a0042ed..212dfcb 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1367,7 +1367,7 @@
     size_t limbs = limbs_N;
     size_t bytes = limbs * sizeof(mbedtls_mpi_uint);
 
-    TEST_EQUAL(limbs_X, 2 * limbs - 1);
+    TEST_EQUAL(limbs_X, 2 * limbs);
     TEST_EQUAL(limbs_res, limbs);
 
     TEST_EQUAL(mbedtls_mpi_mod_modulus_setup(