fdo#39135: Prevent integer overflow & update the character during loop.

These two errors in the code contributed to the reported bug.  Let's
fix them.

Change-Id: If82a1bd4d1e27145b48e722b30388cc9dc4a8a6e
Reviewed-on: https://gerrit.libreoffice.org/2007Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/core/tool/reffind.cxx

@@ -155,15 +155,20 @@ void ExpandToTextR1C1(const sal_Unicode* p, xub_StrLen nLen, xub_StrLen& rStartP
                     if (c == '\'')
                         break;
                 }
+                if (rStartPos == 0)
+                    break;
             }
             else if (c == ']')
             {
                 // Skip until the opening braket.
                 for (--rStartPos; rStartPos > 0; --rStartPos)
                 {
+                    c = p[rStartPos];
                     if (c == '[')
                         break;
                 }
+                if (rStartPos == 0)
+                    break;
             }
             else if (!IsText(c))
             {