Use std::string(cstring, strnlen(cstring, max_length)) instead of
trying to remove extra null characters manually, which is a bit
ugly and error prone.
And indeed, the original code contained a bug which would cause
extra NULLs to not be removed at all if the string did not
end with a NULL -- causing issues down the road when constructing
paths for sub-entries.
gcc complains that the printf %x formatting instruction expects an
'unsigned int' but we pass a 'size_t'. We add the 'z' length formatting
specifier used for 'size_t'
Fixes warning:
```
Source/Core/DiscIO/NANDImporter.cpp:55:17: warning: format specifies type 'unsigned long' but the argument has type 'u64' (aka 'unsigned long long') [-Wformat]
file.GetSize(), NAND_BIN_SIZE);
^~~~~~~~~~~~~~
1 warning generated.
```
