[python] Fix getting section contents.

The returnvalue was handled as c_char_p which ment that ctypes
handled it as a NUL-terminated string making it cut the contents
at first NUL (or even worse - overrunning the buffer if it doesn't
contain a NUL).

Differential Revision: http://reviews.llvm.org/D3474



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207199 91177308-0d34-0410-b5e6-96231b3b80d8