Fix #34: Do not try to use a va_list twice

[kjbracey]

If the prefix function was set, mbed_vtracef would pass ap to vsnprintf to determine the output length, then use ap again for the actual output, running off the real arguments.

Create a copy of ap for the initial scan. (Thank you to C99, who added va_copy. Would have been stuck without it.)

[jupe]

missing test :)

[yogpan01]

Verified with mbed-client-testapp on Linux , works fine +1

[jupe]

thanks :)

[kjbracey]

Expanded the existing prefix function test to take 2 arguments.