Fix a coverity-discovered NULL pointer dereference.

• CID 1372598: Null pointer dereferences (FORWARD_NULL)

/lib/libefivar/efivar-dp-parse.c: 3612 in UefiDevicePathLibConvertTextToDeviceNode()

Dereferencing null pointer "FromText".

When ported from Tiano core, I commented this out with an ifdef. That
was in error because we're supposed to fallback to a filepath when
nothing else patches. Instead, restore the original code, but fix
DevPathFromTextFilePath to cope with the conversion to narrow
strings. Also, fix the off-by-one error in the size of the memory it
allocates.

The off by one error is documented in Tiano core bug
https://bugzilla.tianocore.org/show_bug.cgi?id=441

CID: 1372598
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