NANDImporter: Construct strings correctly

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.

Source/Core/DiscIO/NANDImporter.h

@@ -25,7 +25,7 @@ private:
 #pragma pack(push, 1)
   struct NANDFSTEntry
   {
-    u8 name[12];
+    char name[12];
     u8 mode;   // 0x0C
     u8 attr;   // 0x0D
     u16 sub;   // 0x0E