[GRASS5] [bug #3960] (grass) r.watershed: fix NULL support

this bug's URL: http://intevation.de/rt/webrt?serial_num=3960
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Subject: r.watershed: fix NULL support

Platform: GNU/Linux/i386
grass obtained from: CVS
grass binary for platform: Compiled from Sources
GRASS Version: 2005.11.16

r.watershed still puts 0 where it should put NULL (in the streams
and basins output maps)

Maciek

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