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 b0cc02ed86e3586ab92cc1082fb97b94cd5f584f did
for issue #186.

highlighters/main/main-highlighter.zsh

@@ -311,7 +311,7 @@ _zsh_highlight_main_highlighter_highlight_dollar_string()
     (( k = j + 1 ))
     case "$arg[$i]" in
       "\\") style=$ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]
-            for (( c = i + 1 ; c < end_pos - start_pos ; c += 1 )); do
+            for (( c = i + 1 ; c <= end_pos - start_pos ; c += 1 )); do
               [[ "$arg[$c]" != ([0-9xXuUa-fA-F]) ]] && break
             done
             AA=$arg[$i+1,$c-1]

highlighters/main/test-data/dollar-quoted3.zsh

@@ -0,0 +1,39 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2015 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+# Similar to double-quoted2.zsh
+ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]=$unused_highlight
+# This test checks that the '1' gets highlighted correctly.  Do not append to the BUFFER.
+BUFFER=": \$'\xa1"
+
+expected_region_highlight=(
+  "3 4 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # $'
+  "5 8 $ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]" # \xa1
+)

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'