editeng: avoid autocorrect crash

Happened for me with nFndPos = 30 and nEndPos = 20, and a rTxt that
probably contained "->" twice.

Change-Id: I534c60a5904249e25fdefe45639b4512e6f138c5
Reviewed-on: https://gerrit.libreoffice.org/11344
Reviewed-by: Németh László <nemeth@numbertext.org>
Tested-by: Michael Stahl <mstahl@redhat.com>

editeng/source/misc/svxacorr.cxx

@@ -2833,6 +2833,10 @@ const SvxAutocorrWord* SvxAutocorrWordList::WordMatches(const SvxAutocorrWord *p
                         ;
                     if (bWasWordDelim) nFndPos++;
                 }
+                if (nEndPos + extra_repl <= nFndPos)
+                {
+                    return 0;
+                }
                 // store matching pattern and its replacement as a new list item, eg. "i18ns" -> "internationalizations"
                 OUString aShort = rTxt.copy(nFndPos, nEndPos - nFndPos + extra_repl);