Make ecdsa_verify() return value more explicit
diff --git a/library/ecdsa.c b/library/ecdsa.c
index bdb3567..67774c9 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -134,7 +134,7 @@
     if( mpi_cmp_int( r, 1 ) < 0 || mpi_cmp_mpi( r, &grp->N ) >= 0 ||
         mpi_cmp_int( s, 1 ) < 0 || mpi_cmp_mpi( s, &grp->N ) >= 0 )
     {
-        ret = POLARSSL_ERR_ECP_BAD_INPUT_DATA;
+        ret = POLARSSL_ERR_ECP_VERIFY_FAILED;
         goto cleanup;
     }
 
@@ -168,7 +168,7 @@
 
     if( ecp_is_zero( &R ) )
     {
-        ret = POLARSSL_ERR_ECP_BAD_INPUT_DATA;
+        ret = POLARSSL_ERR_ECP_VERIFY_FAILED;
         goto cleanup;
     }
 
@@ -177,7 +177,7 @@
      */
     if( mpi_cmp_mpi( &R.X, r ) != 0 )
     {
-        ret = POLARSSL_ERR_ECP_BAD_INPUT_DATA;
+        ret = POLARSSL_ERR_ECP_VERIFY_FAILED;
         goto cleanup;
     }