MOBI Output: Fix some sub/super scripts missing

MOBI Output: Fix conversion of some super/sub scripts failling if they
are the first or last element in a paragraph. Fixes #1218858 [Subscripted text is generated by chance (MOBI)](https://bugs.launchpad.net/calibre/+bug/1218858)

src/calibre/ebooks/mobi/mobiml.py

@@ -525,6 +525,8 @@ class MobiMLizer(object):
                     else:
                         break
                 if vbstate.para is not None:
+                    if vbstate.para.text:
+                        vtag.text = vbstate.para.text
                     for child in vbstate.para:
                         vtag.append(child)
                 return