xcursor: check that XDG base directory path is absolute

The spec reads:

> All paths set in these environment variables must be absolute. If an
> implementation encounters a relative path in any of these variables it should
> consider the path invalid and ignore it.

and

> If $XDG_DATA_HOME is either not set or empty, a default equal to
> $HOME/.local/share should be used.

Testing that the path is absolute also entails that is is non-empty.

xcursor/xcursor.c

@@ -647,7 +647,7 @@ XcursorLibraryPath (void)
 	}
 	else {
 		env_var = getenv("XDG_DATA_HOME");
-		if (env_var) {
+		if (env_var && env_var[0] == '/') {
 			pathlen = strlen(env_var) +
 				strlen(CURSORDIR ":" XCURSORPATH) + 1;
 			path = malloc(pathlen);