Fix -loops failure with large-data-transfers (#60)

The -loops option is broken after merging the pull-request #57 (#57).

Here is a sample transcript to reproduce the problem.
hwangcc23-BU401LG:~$ dd 'if=/dev/zero' 'bs=2000' 'count=1000' | tr '\000' 'a' | xclip -i -quiet -l 1

Expect: xclip should exist right after any requestor (ex: `xclip -o`) retrieved data.
What I saw: xclip didn't exist.

Break out the while(1) loop once there is one SelectionRequest completed such that dloop can be decreased by one and compared to sleep for fixing the problem.

xclip.c

@@ -424,9 +424,6 @@ doIn(Window win, const char *progname)
 
 	    if (finished) {
 	    del_requestor(requestor);
-	    }
-
-	    if (!requestors) {
 	    break;
 	    }
 	}