sw: avoid calling convertToString() with unsupported encodings

Change-Id: If0b589c884cd306ca7c94d3366af66276a120176
fdo#78758: sw: RTF export: don't export multiple \fldrst for one hyperlink

Ensure that we export only one \fldresult per hyperlink by doing that in
StartURL() and EndURL(); the TextINetFormat() is called once per text
portion.  This shouldn't cause problems as there can't be anything
between the end of the \field group and the start of \fldresult anyway.

Replace the annoying call to EndURL() from EndRun() with a special case
in EndURL() to store things in the right buffer (hopefully).

(somehow this is regression from c4498251cb7181a9f272b0720f398597c0daef09)

Change-Id: I209ea7a384fb1cb5d1505a70ecc4a4536bbf26a2