Fix UI crash in case trace is less than 50ns
Fix UI crash in case trace is less than 50ns

If a trace is very short (for example, it contains a single instant
event), `stepSecNs` is 0 and load can be NULL.

Bug: 224754227
Change-Id: I395137c019d23a616888cba76247c80e31ff862a

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto