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

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

Addendum to commit d635ce0c.

dll/cpl/sysdm/general.c:156:25: warning: variable 'hCreditsDC' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
                    if (hDC == NULL)
                        ^~~~~~~~~~~
dll/cpl/sysdm/general.c:216:25: note: uninitialized use occurs here
                    if (hCreditsDC != NULL) DeleteDC(hCreditsDC);
                        ^~~~~~~~~~

and the same for hLogoDC:

dll/cpl/sysdm/general.c:215:25: note: uninitialized use occurs here
                    if (hLogoDC != NULL) DeleteDC(hLogoDC);
                        ^~~~~~~