examples/kernel: fix warnings issued by Clang

Fix some "unused variable" and "used before set" warnings.
dfs: assert the path is not null in dfs_filesystem_lookup

Clang complains about "Null pointer argument in call to string
comparison function". So assert on it.
pthreads: remove some useless variables
finsh: add check on converting octal numbers

The digit in octal numbers should with in 0~7. Check on it in
token_proc_number. This issue is found by Clang.
msh: remove usless ptr set

The ptr is set in the following code. So remove the first one.
minilibc: no double braces in if statement
Merge pull request #262 from grissiom/misc-fix

Misc fix
[bsp/taihu] fix the link error

The taihu.lds hardcoded the path of start_gcc.o. Change it as the layout
of build/ has change by commit a03816ef9aa583bb.
Merge pull request #263 from grissiom/fix-taihu

[bsp/taihu] fix the link error
[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