Currently the unknown GPU start is indicated by a -1 value, but it was
maximum int value previously. While the assert check is valid for newly
created traces, it will fire off if an older trace is loaded.
Temporarily disabling the check (effectively until only 0.6 traces are
supported) fixes the problem, as the max int value (0x7f...) has its
high bits removed and the low bytes will be sign extended during number
reconstruction, making it -1, as intended.
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