fips-prf: Add explicit bound check to avoid GCC 12 compile warning

fips-prf: Add explicit bound check to avoid GCC 12 compile warning

GCC assumes this->b is zero (or may be zero) and spits out the following
warning (or error with -Werror):

src/libstrongswan/plugins/fips_prf/fips_prf.c:124:12: error: array subscript 18446744073709551615 is above array bounds of ‘uint8_t[<U8090>]’ {aka ‘unsigned char[<U8090>]’} [-Werror=array-bounds]
  124 |         one[this->b - 1] = 0x01;
      |         ~~~^~~~~~~~~~~~~