Security hardening: missing integer overflow check in curl_load_library()

Summary A missing integer overflow check was identified in `lib/system_win32.c::curl_load_library()` when calculating the buffer size for a DLL path. On 32-bit Windows builds, the unchecked size calculation can wrap around, resulting in an undersized heap allocation followed by unbounded string …

UINT systemdirlen = GetSystemDirectory(NULL, 0);
if(systemdirlen) {
  size_t filenamelen = _tcslen(filename);
  TCHAR *path = curlx_malloc(sizeof(TCHAR) *
                             (systemdirlen + 1 + filenamelen));
  if(path && GetSystemDirectory(path, systemdirlen)) {
    _tcscpy(path + …