Add ASSERT() conditions to UEFI Library Print() and AsciiPrint() functions if gST...

Add ASSERT() conditions to UEFI Library Print() and AsciiPrint() functions if gST->ConOut is NULL.
Add ASSERT() conditions to UEFI Library ErrorPrint() and AsciiErrorPrint() functions if gST->StdErr is NULL.
Add ASSERT() conditions to UEFI Library PrintXY() and AsciiPrintXY() gST->ConsoleOutputHandle is NULL.
Update Print(), AsciiPrint(), ErrorPrint(), AsciiErrorPrint() to return 0 if the Simple Text Output Protocol OutputString() call returns an error.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10576 6f19259b-4bc3-4df7-8a09-765794883524