Turns out that .xlsx files can have multiple link tables, add a test file which shows this (support to follow)

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1611878 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/testcases/org/apache/poi/xssf/model/TestExternalLinksTable.java

@@ -81,4 +81,11 @@ public final class TestExternalLinksTable {
         assertEquals("Defines", name.getSheetName());
         assertEquals("$A$1", name.getRefersToFormula());
     }
+    
+    @Test
+    public void readWithReferencesToTwoExternalBooks() {
+        XSSFWorkbook wb = XSSFTestDataSamples.openSampleWorkbook("ref2-56737.xlsx");
+        
+        // TODO Fix so we can see both of them...
+    }
 }