win32: add more clipboard data checks to avoid crash

win32: add more clipboard data checks to avoid crash

It may happen that the received clipboard data is empty, but
if it's of type image/bmp, gtk+ will crash:

gdk_property_change: 00030AD4 GDK_SELECTION image/bmp REPLACE 8*0 bits:
... delayed rendering
gdk_selection_send_notify_for_display: 00030AD4 CLIPBOARD image/bmp
GDK_SELECTION (no-op)
_gdk_win32_selection_convert_to_dib: 1252003C image/bmp

Program received signal SIGSEGV, Segmentation fault.
0x749a9f40 in msvcrt!memmove () from C:\Windows\syswow64\msvcrt.dll

Thread 1 (Thread 2248.0x1b34):
target=0xc07b) at gdkselection-win32.c:1292
at gdkevents-win32.c:3498
wparam=8, lparam=0) at gdkevents-win32.c:232
message=773, wparam=8, lparam=0)
    at gdkevents-win32.c:263
C:\Windows\syswow64\user32.dll
C:\Users\rugoosse\AppData\Local\virt-viewer\bin\libpangocairo-1.0-0.dll
wparam=0, lparam=-1687549457)
    at gdkevents-win32.c:248
C:\Users\rugoosse\AppData\Local\virt-viewer\bin\libpangocairo-1.0-0.dll

https://bugzilla.gnome.org/show_bug.cgi?id=728745