branch-obs: set DEV_RELEASE to 0 when branching a non dev release

Otherwise this variable is undefined but used later, thus the script
fails with:

./release-scripts/branch-obs: line 82: [: -ne: unary operator expected

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

release-scripts/branch-obs

@@ -2,6 +2,7 @@
 
 set -e
 
+DEV_RELEASE=0
 DRY_RUN=0
 RUN=
 
@@ -162,4 +163,4 @@ obs-complete)
 *)
     echo Invalid command
     exit 1
-esac
\ No newline at end of file
+esac