[mempool] reset the thread->error before suspending thread

Time out result and other errors is recorded in thread->error. Dirty
error will screw up the error handling code after the thread has been
wake up.
Merge pull request #266 from grissiom/fix-mempool

[mempool] reset the thread->error before suspending thread
Add LodePNG library.
Merge branch 'master' of https://github.com/RT-Thread/rt-thread
[External] Merge LodePNG and TJPGD into RT-Thread GUI group in building script.
[bsp/stm32f40x] fix the uart3
Merge pull request #269 from wzyy2/master

[bsp/stm32f40x] fix the uart3
[bsp/stm32f40x] code style change

TAB to 4 space
Update emac.c

Fix PHY chip(dp83848c) init failed without Active Network.
Merge pull request #273 from yuht/patch-2

[bsp/lpc176x] Update emac.c
Merge pull request #272 from wzyy2/master

[bsp/stm32f40x] code style change
[bsp/stm32f40x] code cleanup.
Merge branch 'master' of https://github.com/RT-Thread/rt-thread