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).

Author: mmilenko

rust-mode.el

@@ -80,7 +80,8 @@ Like `looking-back' but for fixed strings rather than regexps (so that it's not
 
 (defun rust-looking-back-macro ()
   "Non-nil if looking back at an ident followed by a !"
-  (save-excursion (backward-char) (and (= ?! (char-after)) (rust-looking-back-ident))))
+  (if (> (point) 2)
+      (save-excursion (backward-char) (and (= ?! (char-after)) (rust-looking-back-ident)))))
 
 ;; Syntax definitions and helpers
 (defvar rust-mode-syntax-table
@@ -1594,7 +1595,7 @@ This is written mainly to be used as `end-of-defun-function' for Rust."
   (when rust-format-on-save
     (unless (executable-find rust-rustfmt-bin)
       (error "Could not locate executable \"%s\"" rust-rustfmt-bin))))
-  
+
 (defvar rustc-compilation-regexps
   (let ((file "\\([^\n]+\\)")
         (start-line "\\([0-9]+\\)")