emphasis: Fix reading of prev/next rune for multibyte characters

org/testdata/inline.html

@@ -5,6 +5,7 @@ also hard line breaks not followed by a newline get ignored, see \\</li>
 <li><em>.emphasis with dot border chars.</em></li>
 <li><em>emphasis with a slash/inside</em></li>
 <li><em>emphasis</em> followed by raw text with slash /</li>
+<li><strong>emphasis ending with a &#34;difficult&#34; multibyte character 习</strong></li>
 <li>-&gt;/not an emphasis/&lt;-</li>
 <li>links with slashes do not become <em>emphasis</em>: <a href="https://somelinkshouldntrenderaccidentalemphasis.com">https://somelinkshouldntrenderaccidentalemphasis.com</a>/ <em>emphasis</em></li>
 <li><span style="text-decoration: underline;">underlined</span> <strong>bold</strong>  <code class="verbatim">verbatim</code> <code>code</code> <del>strikethrough</del></li>

org/testdata/inline.org

@@ -4,6 +4,7 @@
 - /.emphasis with dot border chars./
 - /emphasis with a slash/inside/
 - /emphasis/ followed by raw text with slash /
+- *emphasis ending with a "difficult" multibyte character 习*
 - ->/not an emphasis/<-
 - links with slashes do not become /emphasis/: [[https://somelinkshouldntrenderaccidentalemphasis.com]]/ /emphasis/
 - _underlined_ *bold*  =verbatim= ~code~ +strikethrough+

org/testdata/inline.pretty_org

@@ -4,6 +4,7 @@
 - /.emphasis with dot border chars./
 - /emphasis with a slash/inside/
 - /emphasis/ followed by raw text with slash /
+- *emphasis ending with a "difficult" multibyte character 习*
 - ->/not an emphasis/<-
 - links with slashes do not become /emphasis/: [[https://somelinkshouldntrenderaccidentalemphasis.com]]/ /emphasis/
 - _underlined_ *bold*  =verbatim= ~code~ +strikethrough+