fix(canal): fix "Index of range error" for unsigned columns

[mefcorvi]

It seems that several errors were introduced in the #399

https://github.com/siddontang/go-mysql/blob/046188b858f9a4b47cdedb11068eb868463fcb75/canal/rows.go#L53-L72

• i in L61 is an index of the row, not an index of the column. I guess index should be used.
• i in L64 is always >= 0 since it's an index of the row. Probably it should be t here.
• Weird return in L69. I don't know why we must stop processing other rows after we've found a first mediumint column.
• Duplicated code in L65 and L71 could be avoided by combining conditions in L61 and L64.

Also I suggest to rename index to columnIdx, t to value.

Fixes #404

[mefcorvi]

@Ryan-Git could you please take a look? Am I right in my assumptions about this code?

[Ryan-Git]

yes, u'r right. what the hell I've written...

[mefcorvi]

@siddontang Could you please advise how can I fix issues with CI? It seems that by some reason travis was not able to install mysql. Any comments on PR also are welcome :-)

[siddontang]

thanks @mefcorvi

#410 tries to fix the CI issue but failed with a data race problem.