UBSan: Avoid an unsigned integer overflow warning

UBSan: Avoid an unsigned integer overflow warning

ext_supp_rates_len would be 0 here, so decrementing it by 2 will result
in unsigned integer overflow even if that result is not actually used
anywhere. Avoid that to get rid of the UBSan warning.

tdls.c:1597:27: runtime error: unsigned integer overflow: 0 - 2 cannot be represented in type 'unsigned long'

Signed-off-by: Jouni Malinen <j@w1.fi>