lib: silence gcc-16 compiler warnings -Wmaybe-uninitialized

[vszakats]

Seen in the 'curl-for-win / Windows gcc zlib-classic (x64)' CI job,
after it got an upstream upgrade from gcc-15 to gcc-16:

lib/http.c:206:6: error: 'out.str' may be used uninitialized [-Werror=maybe-uninitialized]
  206 |   if(header_has_value(&header, &out)) {
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/api.h:188:3: error: 'guard.data' may be used uninitialized [-Werror=maybe-uninitialized]
  188 |   Curl_mapi_enter((g), (m), CURL_MAPI_FN_##fn, (r)) && (m)
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/multi.c:1695:6: note: in expansion of macro 'CURL_MAPI_ENTER'
 1695 |   if(CURL_MAPI_ENTER(&guard, m, multi_poll, &mresult)) {
      |      ^~~~~~~~~~~~~~~

Ref: https://github.com/curl/curl/actions/runs/32626356576/job/97162205296#step:3:4906