Unit test for baud rate check and error message

src/modbus-rtu.c

@@ -1148,7 +1148,7 @@ modbus_t* modbus_new_rtu(const char *device,
 
     /* Check baud argument */
     if (baud == 0) {
-        fprintf(stderr, "baud must not be zero\n");
+        fprintf(stderr, "The baud rate value must not be zero\n");
         errno = EINVAL;
         return NULL;
     }

tests/unit-test-client.c

@@ -590,7 +590,10 @@ int main(int argc, char *argv[])
 
     /* Test init functions */
     printf("\nTEST INVALID INITIALIZATION:\n");
-    ctx = modbus_new_rtu(NULL, 0, 'A', 0, 0);
+    ctx = modbus_new_rtu(NULL, 1, 'A', 0, 0);
+    ASSERT_TRUE(ctx == NULL && errno == EINVAL, "");
+
+    ctx = modbus_new_rtu("/dev/dummy", 0, 'A', 0, 0);
     ASSERT_TRUE(ctx == NULL && errno == EINVAL, "");
 
     ctx = modbus_new_tcp_pi(NULL, NULL);