Check for argc in mpsc_test to avoid Segmentation Fault

[kudraw]

mpsc_test doesn't check argc argument of main, while the program require at least argv[1] (capacity of the queue).
This cause the program to Segmentation Fault when launched without arguments.

Just check argc to avoid that, and print a small syntax help for the user.

Small patch is attached. Hope that can be useful. Thank you for the code provided.

argv.patch.gz