core: fix 2 compiler warnings

core: fix 2 compiler warnings

core.c:2361:35: warning: comparison of integers of different signs: 'int' and
      'unsigned long' [-Wsign-compare]
        if (header_len < 0 || header_len >= sizeof(buf)) {
                              ~~~~~~~~~~ ^  ~~~~~~~~~~~
core.c:2370:44: warning: comparison of integers of different signs: 'int' and
      'unsigned long' [-Wsign-compare]
        if (text_len < 0 || text_len + header_len >= sizeof(buf)) {
                            ~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~

Signed-off-by: Ludovic Rousseau <ludovic.rousseau@free.fr>