I've updated it to 5.2.1 and cleaned up a bit. The new version is here:
http://www.geht.de/netbsd/pkgsrc/dmalloc-521.diff.bz2

changes since 5.2.0:
#  Fixed problems with arguments to the various allocation macros in 
dmalloc.h were not surrounded by parens (). This meant that the calloc 
macro could produce bad arithmetic because of operator ordering. Rookie 
mistake.
# Fixed problem with the strdup function being defined as void * instead of 
char *. Also, there was a char ** and void ** bug in the function which 
caused some crashes on some architectures.