[TimeProfiler] Reset variable to nullptr

Otherwise we'll hit a spurious assert failure when we reset and then
reinitialize TimeProfiler on the same thread, as can happen when e.g.
using LLD as a library and running it multiple times in the same
process.

Makes `lld/test/MachO/time-trace.s` pass with `LLD_IN_TEST=2`, which
runs the linker twice in the same process and exposed the issue.

Reviewed By: MaskRay, mehdi_amini

Differential Revision: https://reviews.llvm.org/D112880