don't clobber src/tgt when description line is found

Converting Medutx1.01 from the UTX website, the output would not have languages marked, anywhere. It turned out that the src/tgt values were getting turned into empty strings when the description heading was read.

I haven't reviewed this intensively, but this fixed the problem for me.
Merge pull request #2 from garfieldnate/patch-1

don't clobber src/tgt when description line is found