Is this the cause of the "vertical strip" marsh generation bug?

This is more of a question than an actual pull request- I haven't had a chance to test anything, just thought this line looked suspicious. Pull request format helps for pointing out a single line of code.

This would explain the vertical strips of marshes that people are reporting, though. A marsh biome located on the X=Z diagonal (lower-left to upper-right, going through 0,0) would result in the entire Z column getting erroneously marsh generated. 

Would a more thorough fix require adding additional code to keep the number and timing of calls to rand.nextInt() the same?

My apologies if this was already known. I understand you're planning on replacing marshes for the 4.0 release. A fix that could clean up the buggy marsh terrain without reshuffling the biomes of a pre-existing map would be appreciated, however.