back-dollar-quoted-argument: Correct an off-by-one.

Highlight the last character of a «\xHH» escape when it is the last thing in
LBUFFER.  This is similar to what b0cc02ed86 did
for issue #186.

highlighters/main/test-data/double-quoted2.zsh

@@ -28,6 +28,7 @@
 # vim: ft=zsh sw=2 ts=2 et
 # -------------------------------------------------------------------------------------------------
 
+# Similar to dollar-quoted3.zsh
 # This test checks that the 'r' gets highlighted correctly.  Do not append to the BUFFER.
 BUFFER=': "foo$bar'