Fix SIGSEGV while compiling dthelp on 64 bit systems.

Fix SIGSEGV while compiling dthelp on 64 bit systems.

Patch from Ulrich Wilkens <mail@uwilkens.de>

I have a little patch for a problem that I found when I tried to
compile dthelp on 64bit FreeBSD. It could also be a problem on other
64bit systems.  The problem is that the program context compiles but
fails running with segmentation fault.

context uses the function m_malloc() which is missing a correct
prototype sometimes. Then it's treated to return int instead of void *
. On 64bit systems this cuts off the higher 32 bits because void * is
64bit whereas int is only 32bit.