[SYSDM][USERINIT] Fix uninitialized variables warnings detected by Clang. (#3619)

[SYSDM][USERINIT] Fix uninitialized variables warnings detected by Clang. (#3619)
CORE-17545

Addendum to commit d635ce0c.

- Move the HDC variables initialization via function calls, out of
  the variables declaration block.

- Fix warnings (and identical for base/system/userinit/livecd.c):

dll/cpl/sysdm/general.c:72:9: warning: variable 'hLogo' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
    if (hDC == NULL || hDCLogo == NULL || hDCMask == NULL)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
dll/cpl/sysdm/general.c:130:9: note: uninitialized use occurs here
    if (hLogo != NULL) DeleteObject(hLogo);
        ^~~~~

and similar for hMask too:

dll/cpl/sysdm/general.c:129:9: note: uninitialized use occurs here
    if (hMask != NULL) DeleteObject(hMask);
        ^~~~~