Fix a line_boundaries() crash when there aren't any tags.
Fix a line_boundaries() crash when there aren't any tags.

pombredanne/dxr,kleintom/dxr,srenatus/dxr,pelmers/dxr,KiemVM/Mozilla--dxr,jbradberry/dxr,erikrose/dxr,bozzmob/dxr,srenatus/dxr,srenatus/dxr,srenatus/dxr,bozzmob/dxr,pombredanne/dxr,gartung/dxr,jbradberry/dxr,bozzmob/dxr,nrc/dxr,pelmers/dxr,pelmers/dxr,jbradberry/dxr,pombredanne/dxr,jay-z007/dxr,jay-z007/dxr,kleintom/dxr,srenatus/dxr,jbradberry/dxr,nrc/dxr,nrc/dxr,pombredanne/dxr,kleintom/dxr,gartung/dxr,jay-z007/dxr,pombredanne/dxr,jay-z007/dxr,jay-z007/dxr,kleintom/dxr,pelmers/dxr,bozzmob/dxr,gartung/dxr,erikrose/dxr,nrc/dxr,erikrose/dxr,bozzmob/dxr,erikrose/dxr,KiemVM/Mozilla--dxr,srenatus/dxr,jay-z007/dxr,pelmers/dxr,bozzmob/dxr,jbradberry/dxr,kleintom/dxr,pelmers/dxr,KiemVM/Mozilla--dxr,KiemVM/Mozilla--dxr,jbradberry/dxr,nrc/dxr,gartung/dxr,kleintom/dxr,jay-z007/dxr,pombredanne/dxr,pelmers/dxr,KiemVM/Mozilla--dxr,pombredanne/dxr,erikrose/dxr,gartung/dxr,gartung/dxr,kleintom/dxr,bozzmob/dxr,nrc/dxr,gartung/dxr,jbradberry/dxr,KiemVM/Mozilla--dxr