avoid calling OUString::copy() with length being -1

Which asserts in dbgutil build.

Change-Id: I0f8205ae98c5a57c430b7d8a574e4c0019855841

basic/source/comp/scanner.cxx

@@ -491,9 +491,8 @@ bool SbiScanner::NextSym()
             if( *pLine == cSep )
             {
                 pLine++; nCol++;
-                if( *pLine != cSep || cSep == ']' ) break;
-            } else aError = OUString(cSep), GenError( SbERR_EXPECTED );
-        }
+                if( *pLine != cSep || cSep == ']' )
+                {
                     // If VBA Interop then doen't eat the [] chars
                     if ( cSep == ']' && bVBASupportOn )
                         aSym = aLine.copy( n - 1, nCol - n  + 1);
@@ -510,6 +509,10 @@ bool SbiScanner::NextSym()
                     aSym = aSymBuf.makeStringAndClear();
                     if( cSep != ']' )
                         eScanType = ( cSep == '#' ) ? SbxDATE : SbxSTRING;
+                    break;
+                }
+            } else aError = OUString(cSep), GenError( SbERR_EXPECTED );
+        }
     }
     // invalid characters:
     else if( ( *pLine & 0xFF ) >= 0x7F )