Correctly handle null termination of comments in spexical files

src/spexipy/spexical.py

@@ -72,6 +72,8 @@ class SpeXICAL:
             self.pixel_mask = self._load_frame(np.uint16)
 
         self.comment = self._file.read(self.file_size - self._min_size)
+        if self.comment[-1] == b'\x00':
+            self.comment = self.comment[:-1]
 
         self._file.close()
         self._file = None
@@ -128,6 +130,7 @@ class SpeXICAL:
         self._store_frame(self.pixel_mask)
 
         self._file.write(self.comment)
-        self._file.write(b'\x00')
+        if self.comment[-1] != b'\x00':
+            self._file.write(b'\x00')
         self._file.close()
         self._file = None