oauth.c: fix compiler warning

make[1]: Entering directory '/home/ruben/src/collectd'
  CC       src/utils/oauth/liboauth_la-oauth.lo
src/utils/oauth/oauth.c: In function ‘get_assertion’:
src/utils/oauth/oauth.c:283:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
  283 |   else if (status >= buffer_size)
      |                   ^~