v.kernel: valid file descriptors are non-negative

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@50084 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.kernel/main.c

@@ -80,7 +80,7 @@ int main(int argc, char **argv)
     struct Flag *flag_o, *flag_q, *flag_normalize, *flag_multiply;
 
     struct Map_info In, Net, Out;
-    int fdout = 0, maskfd = 0;
+    int fdout = -1, maskfd = -1;
     int node_method, kernel_function;
     int row, col;
     struct Cell_head window;