radix: add debug validation to assist scan-build

radix: add debug validation to assist scan-build

util-radix-tree.c:595:34: warning: Access to field 'stream' results in a dereference of a null pointer (loaded from field 'prefix') [core.NullDereference]
        if ((temp = (stream[i] ^ bottom_node->prefix->stream[i])) == 0) {
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
util-radix-tree.c:717:30: warning: Access to field 'stream' results in a dereference of a null pointer (loaded from field 'prefix') [core.NullDereference]
        if (SC_RADIX_BITTEST(bottom_node->prefix->stream[differ_bit >> 3],
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
./util-radix-tree.h:27:34: note: expanded from macro 'SC_RADIX_BITTEST'
 #define SC_RADIX_BITTEST(x, y) ((x) & (y))
                                 ^
2 warnings generated.