src/rrd_daemon.c: "comparison between pointer and zero character constant"

src/rrd_daemon.c: "comparison between pointer and zero character constant"

rrd_daemon.c: In function ‘handle_request_list’:
rrd_daemon.c:2587:22: warning: comparison between pointer and zero character constant [-Wpointer-compare]
   } while (start_ptr != '\0');
                      ^~
rrd_daemon.c:2587:12: note: did you mean to dereference the pointer?
   } while (start_ptr != '\0');

I thought this might be related to what travis picked up with valgrind
here, but I couldn't reproduce it.  Also I don't think this commit fixes a
bug; all it's doing is cleaning up the compiler warning.

Starting rrdcached...
OK: empty directory ./list1_dir returns nothing
==23957== LEAK SUMMARY:
==23957==    definitely lost: 0 bytes in 0 blocks
==23957==    indirectly lost: 0 bytes in 0 blocks
==23957==      possibly lost: 0 bytes in 0 blocks
==23957==    still reachable: 14,712 bytes in 6 blocks
==23957==         suppressed: 0 bytes in 0 blocks
==23957==
==23957== For counts of detected and suppressed errors, rerun with: -v
==23957== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)
FAILED: (rc=valgrind error) single file /list1.rrd
FAIL: list1