Properly check existence of a connection type.

Substituting "/dev/null/nonexistent" for an empty $Connection,
of course, does generally work, but is somewhat ugly (esp. in that
the string is appended to $CONN_DIR, so the resulting path may
theoretically exist).

Instead, substitute an empty string for the complete path if $Connection
is empty: an empty string can never be readable.
Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>

src/lib/globals

@@ -109,7 +109,7 @@ load_profile() {
     if [[ -z $Interface ]]; then
         exit_error "Profile '$1' does not specify an interface"
     fi
-    if [[ ! -r "$CONN_DIR/${Connection:-/dev/null/nonexistent}" ]]; then
+    if [[ ! -r "${Connection:+$CONN_DIR/$Connection}" ]]; then
         exit_error "Profile '$1' does not specify a valid connection"
     fi
     if [[ -x "$PROFILE_DIR/interfaces/$Interface" ]]; then