Fix empty capture group reference

Searching for something like (foo)|(bar) and replacing with $1$2 would fail
assertion in appendCopy in include/rtl/ustrbuf.hxx, because beginIndex is
negative (-1), because one of the references is always empty (SearchResult
at TextSearch::ReplaceBackReferences() has both startOffset and startOffset
equal to -1).

In this case, we should simply return an empty string.

Change-Id: Ibf91b1d17ab21c279cfcdc31e84d0c2eae567a53
Reviewed-on: https://gerrit.libreoffice.org/62248
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

unotools/source/i18n/textsearch.cxx

@@ -329,7 +329,11 @@ void TextSearch::ReplaceBackReferences( OUString& rReplaceStr, const OUString &r
                         {
                             sal_Int32 nSttReg = rResult.startOffset[j];
                             sal_Int32 nRegLen = rResult.endOffset[j];
-                            if( nRegLen > nSttReg )
+                            if (nSttReg < 0 || nRegLen < 0) // A "not found" optional capture
+                            {
+                                nSttReg = nRegLen = 0; // Copy empty string
+                            }
+                            else if (nRegLen >= nSttReg)
                             {
                                 nRegLen = nRegLen - nSttReg;
                             }