Change ecp_mul() prototype to allow randomization

(Also improve an error code while at it.)
diff --git a/tests/suites/test_suite_ecp.data b/tests/suites/test_suite_ecp.data
index ce92633..55a3453 100644
--- a/tests/suites/test_suite_ecp.data
+++ b/tests/suites/test_suite_ecp.data
@@ -50,7 +50,7 @@
 ecp_small_sub:0:"14":"11":0:"14":"36":0:27:30
 
 ECP small multiplication negative
-ecp_small_mul:-1:0:0:0:POLARSSL_ERR_ECP_GENERIC
+ecp_small_mul:-1:0:0:0:POLARSSL_ERR_ECP_BAD_INPUT_DATA
 
 ECP small multiplication #0
 ecp_small_mul:0:1:0:0:0
@@ -101,7 +101,7 @@
 ecp_small_mul:2:0:20:01:0
 
 ECP small multiplication too big
-ecp_small_mul:-1:0:0:0:POLARSSL_ERR_ECP_GENERIC
+ecp_small_mul:-1:0:0:0:POLARSSL_ERR_ECP_BAD_INPUT_DATA
 
 ECP small check pubkey #1
 ecp_small_check_pub:1:1:0:POLARSSL_ERR_ECP_GENERIC