lib: fix compiler warnings (gcc)

[vszakats]

Seen when setting ENABLE_DEBUG=ON and -DDEBUGBUILD for mingw-w64
gcc 13.2.0 CMake unity builds in 'Release' configurations.

curl/lib/curl_gethostname.c:71:5: error: 'strncpy' specified bound 1025 equals destination size [-Werror=stringop-truncation]
   71 |     strncpy(name, force_hostname, namelen);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from curl/_bld/lib/CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c:175:
In function 'hostcache_timestamp_remove',
    inlined from 'Curl_hash_clean_with_criterium' at curl/lib/hash.c:265:19,
    inlined from 'Curl_hash_clean_with_criterium' at curl/lib/hash.c:247:1,
    inlined from 'hostcache_prune' at curl/lib/hostip.c:228:3,
    inlined from 'Curl_hostcache_prune' at curl/lib/hostip.c:256:21:
curl/lib/hostip.c:205:12: error: 'now' may be used uninitialized [-Werror=maybe-uninitialized]
  205 |     time_t age = prune->now - c->timestamp;
      |            ^~~
curl/lib/hostip.c: In function 'Curl_hostcache_prune':
curl/lib/hostip.c:241:10: note: 'now' was declared here
  241 |   time_t now;
      |          ^~~
In function 'hostcache_timestamp_remove',
    inlined from 'fetch_addr' at curl/lib/hostip.c:310:8:
curl/lib/hostip.c:205:23: error: 'user.now' may be used uninitialized [-Werror=maybe-uninitialized]
  205 |     time_t age = prune->now - c->timestamp;
      |                  ~~~~~^~~~~
curl/lib/hostip.c: In function 'fetch_addr':
curl/lib/hostip.c:304:33: note: 'user' declared here
  304 |     struct hostcache_prune_data user;
      |                                 ^~~~
In file included from curl/_bld/lib/CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c:40:
curl/lib/cf-socket.c: In function 'cf_socket_send':
curl/lib/cf-socket.c:1294:10: error: 'c' may be used uninitialized [-Werror=maybe-uninitialized]
 1294 |     if(c >= ((100-ctx->wblock_percent)*256/100)) {
      |        ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
curl/lib/cf-socket.c:1292:19: note: 'c' was declared here
 1292 |     unsigned char c;
      |                   ^
In file included from curl/_bld/lib/CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c:364:
In function 'tftp_state_timeout',
    inlined from 'tftp_multi_statemach' at curl/lib/tftp.c:1230:27:
curl/lib/tftp.c:1208:5: error: 'current' may be used uninitialized [-Werror=maybe-uninitialized]
 1208 |   if(current > state->rx_time + state->retry_time) {
      |     ^
curl/lib/tftp.c: In function 'tftp_multi_statemach':
curl/lib/tftp.c:1192:10: note: 'current' was declared here
 1192 |   time_t current;
      |          ^~~~~~~

Ref: https://ci.appveyor.com/project/curlorg/curl/builds/49792835/job/91c8dj5qb36spfe0#L112
Ref: https://github.com/curl/curl/actions/runs/9082968838/job/24960616145#step:12:62

Ref: #13592
Closes #13643