RTF Output: Fix bug that broke conversion to RTF when the input document contains <img> tags with no src attribute.

src/calibre/ebooks/rtf/rtfml.py

@@ -234,6 +234,8 @@ class RTFMLizer(object):
         # Process tags that need special processing and that do not have inner
         # text. Usually these require an argument
         if tag == 'img':
+            src = elem.get('src')
+            if src:
                 src = os.path.basename(elem.get('src'))
                 block_start = ''
                 block_end = ''