Found with Clang's `scan-build` tool.

When get_timer() is called with `reset` set to 1, the value of
t->start.tv_sec is used as a rvalue without being initialized first.
This is relatively harmless because the result of get_timer() is not
used by the callers when called in "reset mode". However, scan-build
prints a warning.

Silence the warning by only calculating the delta on non-reset runs,
returning zero otherwise.