Add nil check when following `Else` in ast

Fixes #137

source/source.go

@@ -120,6 +120,7 @@ func (s *Searcher) NextLines(fname string, line int) (lines []int, err error) {
 					x = stmt
 					continue
 				}
+				if x.Else != nil {
 					pos := s.fileset.Position(x.Else.Pos())
 					ast.Inspect(x, func(n ast.Node) bool {
 						if found {
@@ -138,6 +139,7 @@ func (s *Searcher) NextLines(fname string, line int) (lines []int, err error) {
 					})
 				}
 			}
+		}
 
 	// Follow case statements.
 	//