Looks like the NS guys f*d up; segfaults aren't supposed to happen in good releases. You should report it as a bug.
But, to answer your question, try one of these:
strace /usr/local/bin/ns sim.tcl
gdb --args /usr/local/bin/ns sim.tcl
# type run, then bt
valgrind /usr/local/bin/ns sim.tcl
Then use your Googling skills to figure out what's wrong, based on the information those things return.
Also, double check your input file. You might be passing a bad value to update_position().