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
gdk/win32: remove extra allocation for \r removal

Although I can't find explicit documentation for clipboard pointer, it
seems to be possible to modify clibpoard memory without side-effects.

According to MSDN,
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366596%28v=vs.85%29.aspx

"The global and local functions are supported for porting from 16-bit
code, or for maintaining source code compatibility with 16-bit
Windows. Starting with 32-bit Windows, the global and local functions
are implemented as wrapper functions that call the corresponding heap
functions using a handle to the process's default heap."

"Memory objects allocated by GlobalAlloc and LocalAlloc are in private,
committed pages with read/write access that cannot be accessed by other
processes. Memory allocated by using GlobalAlloc with GMEM_DDESHARE is
not actually shared globally as it is in 16-bit Windows. This value has
no effect and is available only for compatibility. "

https://bugzilla.gnome.org/show_bug.cgi?id=711553
win32: do not crash on invalid utf8 conversion

g_utf8_to_utf16() is not guaranteed to succeed. Check the error
and return if it failed.

https://bugzilla.gnome.org/show_bug.cgi?id=696232
gdk/win32: VK_SNAPSHOT maps to GDK_Print

Also, I am not sure the above VK_PRINT -> GDK_Print mapping is
correct, but it doesn't hurt yet.

https://bugzilla.gnome.org/show_bug.cgi?id=686170