The error report timer is non-negative; make it FT_UINT16.

This squelches an assertion.

Use %u to print it, while we're at it.

Change-Id: I79d06c5448a159e3e04930db776a9063c1004c03
Reviewed-on: https://code.wireshark.org/review/3712
Reviewed-by: Guy Harris <guy@alum.mit.edu>