Only set the initial break based on the main program

The previous check was based on hard-coded base addresses, and
didn't work well with prelinked ld.so. Putting the break after ld.so's
data segment will likely cause it to conflict with another library;
on my system it overlapped with libc's PLT. Since FuzzBALL doesn't
know about address space conflicts, this just led to a weird null
dereference crash.