Number scanner: accept negative year ISO 8601 input with 'T' time separator

This is valid input: -1999-11-23T12:34:56

Change-Id: I0464b623b76fb55c1d2b848d0db9278e9589a4d8

svl/source/numbers/zforfind.cxx

@@ -2544,9 +2544,10 @@ bool ImpSvNumberInputScan::ScanMidString( const OUString& rString,
             }
             else if (nPos == 0 && rString.getLength() == 1 && MayBeIso8601())
             {
-                if (nStringPos == 5 && rString[0] == 'T')
+                if (    (nStringPos == 5 && rString[0] == 'T') ||
+                        (nStringPos == 6 && rString[0] == 'T' && sStrArray[0] == "-"))
                 {
-                    // ISO 8601 combined date and time, yyyy-mm-ddThh:mm
+                    // ISO 8601 combined date and time, yyyy-mm-ddThh:mm or -yyyy-mm-ddThh:mm
                     ++nPos;
                 }
                 else if (nStringPos == 7 && rString[0] == ':')