tdf#167648 sw letter spacing: fix division by zero

/sw/source/core/text/portxt.cxx:351:90: runtime error: division by zero
    #0 0x7f7bf4700824 in SwTextPortion::SetSpacing(SwTextFormatInfo&, o3tl::strong_int<int, Tag_TextFrameIndex>, int, short) /sw/source/core/text/portxt.cxx:351:90

(<https://ci.libreoffice.org//job/lo_ubsan/3669/>)

Regression from commit 3c53797210
"tdf#167648 sw letter spacing: implement minimum letter spacing".

Change-Id: I1dbea7ce855c1f4cb0604e6f9b96b875dd8eb646
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/190608
Reviewed-by: László Németh <nemeth@numbertext.org>
Tested-by: Jenkins

sw/source/core/text/portxt.cxx

@@ -344,6 +344,10 @@ sal_uInt16 SwTextPortion::GetMaxComp(const SwTextFormatInfo& rInf) const
 void SwTextPortion::SetSpacing( SwTextFormatInfo &rInf, const TextFrameIndex nBreakPos,
                 const sal_Int32 nSpaces, const sal_Int16 nWidthOf10Spaces )
 {
+    // TODO allow letter spacing and glyph scaling in single word lines, too
+    if ( nSpaces == 0 )
+        return;
+
     SvxAdjustItem aAdjustItem =
         rInf.GetTextFrame()->GetTextNodeForParaProps()->GetSwAttrSet().GetAdjust();
     // width of a single expanded space without letter spacing and glyph scaling