libgis: -1 is a valid compression level (Z_DEFAULT_COMPRESSION)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@61422 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/gis/gisinit.c

@@ -118,7 +118,7 @@ static int gisinit(void)
     G__.little_endian = G_is_little_endian();
 
     zlib = getenv("GRASS_ZLIB_LEVEL");
-    G__.compression_level = (zlib && *zlib && isdigit(*zlib)) ? atoi(zlib) : -1;
+    G__.compression_level = (zlib && *zlib && isdigit(*zlib)) ? atoi(zlib) : -2;
 
     initialized = 1;