better fix for #30646. r=mscott, a=jar.

netwerk/streamconv/converters/mozTXTToHTMLConv.cpp

@@ -1140,8 +1140,10 @@ mozTXTToHTMLConv::ScanTXT(const PRUnichar *text, PRUint32 whattodo,
   // need to reallocate and re-copy the characters already in the out String.
   NS_ASSERTION(inLength, "ScanTXT passed 0 length string");
   if (inLength == 0) {
-    return NS_ERROR_FAILURE;
+    *_retval = nsCRT::strdup(text);
+    return NS_OK;
   }
+
   outString.SetCapacity(inLength * growthRate);
   ScanTXT(text, inLength, whattodo, outString);