Terminate the execl() argument list with (char *)NULL instead of 0.

The later is wrong and may fail on sizeof(char *) != sizeof(int)
machines. The patch has already been sent to and accepted by the
diffutils author, but the file is already off the vendor branch, so
correct it here in the meanwhile too.

contrib/diff/util.c

@@ -210,9 +210,9 @@ begin_output ()
 	      close (pipes[0]);
 	    }
 #ifdef __FreeBSD__
-	  execl (PR_PROGRAM, PR_PROGRAM, "-F", "-h", name, 0);
+	  execl (PR_PROGRAM, PR_PROGRAM, "-F", "-h", name, (char *)NULL);
 #else
-	  execl (PR_PROGRAM, PR_PROGRAM, "-f", "-h", name, 0);
+	  execl (PR_PROGRAM, PR_PROGRAM, "-f", "-h", name, (char *)NULL);
 #endif
 	  pfatal_with_name (PR_PROGRAM);
 	}