Prevents crash when encountering PNGs with utf8 comments

Signed-off-by: Erling A. Jacobsen <linuxcub@email.dk>

diff -ur xv-3.10a-orig/xvpng.c xv-3.10a/xvpng.c
--- xv-3.10a-orig/xvpng.c	2014-01-21 14:56:24.168673164 +0100
+++ xv-3.10a/xvpng.c	2014-01-21 15:01:02.567883070 +0100
@@ -1149,7 +1149,7 @@
 
     for (i = 0; i < _num_text; i++)
       commentsize += strlen(_text[i].key) + 1 +
-                     _text[i].text_length + 2;
+                     (_text[i].text_length ? _text[i].text_length : _text[i].itxt_length) + 2;
 
     if ((pinfo->comment = malloc(commentsize)) == NULL) {
       png_warning(png_ptr,"can't allocate comment string");