ceph plugin: fix scan-build warnings

ceph plugin: fix scan-build warnings

src/ceph.c:284:8: warning: Null pointer argument in call to string comparison function
  (strcmp("filestore", state->stack[state->depth - 2]) == 0) &&
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/x86_64-linux-gnu/bits/string2.h:817:8: note: expanded from macro 'strcmp'
	: __builtin_strcmp (s1, s2)))); })
	  ^~~~~~~~~~~~~~~~~~~~~~~~~
src/ceph.c:285:8: warning: Null pointer argument in call to string comparison function
  (strcmp("journal_wr_bytes", state->stack[state->depth - 1]) == 0) &&
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/x86_64-linux-gnu/bits/string2.h:817:8: note: expanded from macro 'strcmp'
	: __builtin_strcmp (s1, s2)))); })
	  ^~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.