libqcdm: use NULL instead of 0 for empty pointer

When I use clang 3.5 to build it, I got

commands.c:1444:16: error: expression which evaluates to zero treated as a
null
      pointer constant of type 'QcdmResult *' (aka 'struct QcdmResult *')
      [-Werror,-Wnon-literal-null-conversion]
        return FALSE;
               ^~~~~
./utils.h:29:15: note: expanded from macro 'FALSE'
#define FALSE ((u_int8_t) 0)
              ^~~~~~~~~~~~~~
commands.c:1464:20: error: expression which evaluates to zero treated as a
null
      pointer constant of type 'QcdmResult *' (aka 'struct QcdmResult *')
      [-Werror,-Wnon-literal-null-conversion]
            return FALSE;
                   ^~~~~
./utils.h:29:15: note: expanded from macro 'FALSE'
#define FALSE ((u_int8_t) 0)
              ^~~~~~~~~~~~~~
2 errors generated.
make[3]: *** [libqcdm_la-commands.lo] Error 1
make[3]: *** Waiting for unfinished jobs....

Below is the patch that fixes it

libqcdm/src/commands.c

@@ -1441,7 +1441,7 @@ qcdm_cmd_ext_logmask_result (const char *buf, size_t len, int *out_error)
     if (len < 1) {
         qcdm_err (0, "DM command %d response not long enough (got %zu, expected "
                   "at least %d).", DIAG_CMD_EXT_LOGMASK, len, 3);
-        return FALSE;
+        return NULL;
     }
 
     /* Result of a 'set' operation will be only 1 byte in size; result of
@@ -1461,7 +1461,7 @@ qcdm_cmd_ext_logmask_result (const char *buf, size_t len, int *out_error)
         if (len < (masklen + 3)) {
             qcdm_err (0, "DM command %d response not long enough (got %zu, expected "
                       "at least %d).", DIAG_CMD_EXT_LOGMASK, len, masklen + 3);
-            return FALSE;
+            return NULL;
         }
         minlen = masklen + 3;
     }