fix access of element [0] of array even though it sometimes does not exist

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/elements/ReplyThread.js

@@ -142,7 +142,7 @@ export default class ReplyThread extends React.Component {
     // Part of Replies fallback support
     static stripPlainReply(body) {
         const lines = body.split('\n');
-        while (lines[0].startsWith('> ')) lines.shift();
+        while (lines.length && lines[0].startsWith('> ')) lines.shift();
         return lines.join('\n');
     }