fix multi copy step data index

[DreamWuGit]

this PR can be part of #476 when doing multiple copy step in buss mapping , for TxLogFieldTag::Data index should start from zero and consecutively increasing in multi copy log steps. i.e. total copies size is 66 bytes, data index should be monotone increasing by 1 from 0 across all three copy steps.

step	data_start_index	copy range
CopyStep1	0	0 --> 31
CopyStep2	32	32 --> 63
CopyStep2	64	64 -> 65

while old is as below though Data value is correct !

step	data_start_index	copy range
CopyStep1	0	0 --> 31
CopyStep2	0	0 --> 31
CopyStep2	0	0 --> 31

found in issue #511

[icemelon]

should add a test case that could cover the new fix code

[DreamWuGit]

should add a test case that could cover the new fix code

tests of copy_to_log_multi_step cover this, the incoming log test with multiple steps can indirect check this !

[ed255]

LGTM! Thanks for the fix :)

[icemelon]

LGTM