ovmf-native: fix compile issue on new OS like FC27 and Ubuntu 17

ovmf-native: fix compile issue on new OS like FC27 and Ubuntu 17

Fixed this error:
| VfrUtilityLib.cpp:3375:26: error: ISO C++ forbids comparison between
pointer and integer [-fpermissive]
|    if (mStringFileName == '\0' ) {
|                           ^~~~
| VfrUtilityLib.cpp:3398:9: warning: ignoring return value of ‘size_t
fread(void*, size_t, size_t, FILE*)’, declared with attribute
warn_unused_result [-Wunused-result]
|    fread ((char *)StringPtr, sizeof (UINT8), Length, pInFile);
|    ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ../Makefiles/footer.makefile:30: recipe for target 'VfrUtilityLib.o'
failed

[Yocto #12530]

Signed-off-by: Armin Kuster <akuster808@gmail.com>