Avoid UB pointer arithmetic in T1 parsing

Check that the parser cursor is not already at the limit before creating
a pointer to one after it. This allows for safely skipping the 'white'
only if it is actually present.

Bug: https://crbug.com/1258839
8 jobs for t1_truncated_size in 1 minute and 14 seconds (queued for 5 seconds)
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