For the following APIs in PrintLib instance
MdeModulePkg\Library\DxePrintLibPrint2Protocol:
UnicodeVSPrint
UnicodeSPrint
UnicodeVSPrintAsciiFormat
UnicodeSPrintAsciiFormat
AsciiVSPrint
AsciiSPrint
AsciiVSPrintUnicodeFormat
AsciiSPrintUnicodeFormat
The internal function DxePrintLibPrint2ProtocolVaListToBaseList() will be
called to convert a VA_LIST to a BASE_LIST. However, those APIs miss
checking the return value of the internal function.
This commit adds codes to check the return value. If the VA_LIST fails to
be converted to a BASE_LIST, those PrintLib APIs will return 0 and leave
the output 'StartOfBuffer' unmodified.