Multiple fixes for compilation with gcc 6

Multiple fixes for compilation with gcc 6

Multiple instances of non-NULL sanity checks against function
arguments declared as __attribute__((__nonnull__)); gcc 6 now triggers
warnings on these.

dp.c: Remove Unused variable end_instance

linux.c: Fix warnings about dereferencing type-punned pointers: switch
from char * to uint8_t * in various places

Signed-off-by: Steve McIntyre <steve@einval.com>