Avoid a few out-of-bounds reads

* When "omr" is called without any arguments
  + instead just do nothing
* in string_scan_range
* unbounded pdj now works under ASAN.
  + But what did the +5 do there?
* on px/ without arguments
minor cleanup in java: remove a now-useless snprintf

It became useless in 0.9.6-330-g415a77d.
TODO.md: fix the banner for github's markdown parser
fix a buffer underflow in r_core_editor

It could happen if the edited file was empty.
fix instability in r_reg_free_internal

This is a partial revert of 0814a6d4e ("Fix memleak in r_reg_free").