Fix sparse check exception

If a program file is less than 28 bytes,
the checking for sparse header will fire exception,
Since the unpack funciton expect the input data length
to be 28 bytes.

This was accidentally reverted by commit
4d5cb463d2

edlclient/Library/sparse.py

@@ -43,7 +43,10 @@ class QCSparse(metaclass=LogBase):
             self.__logger.addHandler(fh)
 
     def readheader(self):
-        header = unpack("<I4H4I", self.rf.read(0x1C))
+        buf = self.rf.read(0x1C)
+        if len(buf) != 28:
+            return False
+        header = unpack("<I4H4I", buf)
         magic = header[0]
         self.major_version = header[1]
         self.minor_version = header[2]