Fix crash caused by empty span when removing leading newlines

Change-Id: I9045bf1314b15d3672d1837bca29fa4af4c0fdbd

vector/src/main/java/im/vector/app/features/html/EventHtmlRenderer.kt

@@ -277,6 +277,9 @@ class EventHtmlRenderer @Inject constructor(
 
             spans.forEach { span ->
                 val start = text.getSpanStart(span)
+                if (start < 0) {
+                    return@forEach
+                }
                 if (text[start] == '\n') {
                     text.replace(start, start + 1, "")
                 }