thunk: assert nb_fields is valid

thunk.c:91:32: warning: Call to 'malloc' has an allocation size of 0 bytes
        se->field_offsets[i] = malloc(nb_fields * sizeof(int));
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
