Add some negative testing for ecdsa_verify()
diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h
index a277715..259948d 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -120,7 +120,7 @@
  *                  prescribed by SEC1 4.1.4 step 3.
  *
  * \return          0 if successful,
- *                  MBEDTLS_ERR_ECP_BAD_INPUT_DATA if signature is invalid
+ *                  MBEDTLS_ERR_ECP_VERIFY_FAILED if signature is invalid
  *                  or a MBEDTLS_ERR_ECP_XXX or MBEDTLS_MPI_XXX error code
  */
 int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp,