fix crash with empty booknames

Change-Id: Ifadb2129be53530720cb2023cdc0782b6e737f18
(cherry picked from commit 11e73a08)
Reviewed-on: https://gerrit.libreoffice.org/17944Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

sw/source/filter/ww8/ww8scan.cxx

@@ -4202,17 +4202,18 @@ bool WW8PLCFx_Book::MapName(OUString& rName)
         return false;
 
     bool bFound = false;
-    sal_uInt16 i = 0;
-    do
+
+    size_t i = 0;
+    while (i < aBookNames.size())
     {
         if (rName.equalsIgnoreAsciiCase(aBookNames[i]))
         {
             rName = aBookNames[i];
             bFound = true;
+            break;
         }
         ++i;
     }
-    while (!bFound && i < pBook[0]->GetIMax());
     return bFound;
 }