decode: read char-frequency pairs in heap order
decode: read char-frequency pairs in heap order

This solves the issue of SP_EOF being interpreted as 4. It also saves us the
time for building the min heap since it's only done once at compression time.

Signed-off-by: Pang Yan Han <22594c345857f664c61cd411f607c4d40a9d4af9@gmail.com>

yanhan/cs2010,yanhan/cs2010,yanhan/cs2010,yanhan/cs2010