Avoid signaling "Beginning of buffer" in rust-lookng-back-macro

Also save some work if we don't have at least two characters behind us
in the buffer, we know we can't be looking at a macro then. The signal
occurs when one tries 'racer-describe' when positioned over "String",
which eventually ends up calling

(with-temp-buffer
  (insert "<Target=str>")
  (delay-mode-hooks (rust-mode))
  (font-lock-ensure))

This simpler block will also trigger the error

(with-temp-buffer
  (insert "<>")
  (rust-mode)
  (font-lock-ensure))

(font-lock-ensure) call ends up calling (rust-looking-back-macro).

0 files changed, 0 insertions, 0 deletions