And of course the backtrace can be improved when installing the debugging symbols for libc

/lib/i386-linux-gnu/i686/cmov/libpthread.so.0(pthread_setname_np+0x50) [0xf73e0ed0] ../nptl/sysdeps/unix/sysv/linux/pthread_setname.c:49

The exact source can be downloaded from https://packages.debian.org/jessie/libc6 (keep in mind that the original source and the packages have to be downloaded separately, and be combined.

And since it's pretty easy to get the relevant source on a running debian system:

int 
pthread_setname_np (th, name)
     pthread_t th;
     const char *name;
{
  const struct pthread *pd = (const struct pthread *) th;
...
#define FMT "/proc/self/task/%u/comm" 
  char fname[sizeof (FMT) + 8];
  sprintf (fname, FMT, (unsigned int) pd->tid);

Line 49 is the last line, and this is indeed the first part where a member of td is queried.