runat.c: Add an explicit check for snprintf() failure

The check for "outsiz" too large was probably sufficient to
catch failures, since it was cast to an unsigned (size_t).
However, it seems appropriate to add an explicit check for
a failed case (returning -1).

Discussed with: oshogbo
Fixes: 0660de8172cd ("runat: Add a runat(1) utility similar to the Solaris one")