Remove redundant code

Found by clang-check.
(the last one looks like a false-positive to me)

parson.c:995:17: warning: Value stored to 'buf' is never read
                buf += written;
                ^      ~~~~~~~
parson.c:1016:17: warning: Value stored to 'buf' is never read
                buf += written;
                ^      ~~~~~~~
parson.c:1834:24: warning: Array access (via field 'names') results in a null pointer dereference
            if (strcmp(object->names[i], name) == 0) {
                       ^~~~~~~~~~~~~~~~
3 warnings generated.

parson.c

@@ -991,9 +991,6 @@ static int json_serialize_to_buffer_r(const JSON_Value *value, char *buf, int le
             if (written < 0) {
                 return -1;
             }
-            if (buf != NULL) {
-                buf += written;
-            }
             written_total += written;
             return written_total;
         case JSONBoolean:
@@ -1012,9 +1009,6 @@ static int json_serialize_to_buffer_r(const JSON_Value *value, char *buf, int le
             if (written < 0) {
                 return -1;
             }
-            if (buf != NULL) {
-                buf += written;
-            }
             written_total += written;
             return written_total;
         case JSONNull: