go/ast/astutil: do not merge if import path is last line

DeleteNamedImport assumes the import declaration is in the form of:

    import (
      "foo"
    )

If an import path is deleted there might be a blank line-sized hole:

    import (

    )

It'll merge the black hole with the last line to change it to:

    import (
    )

However the import declaration might be in the following form as well:

    import (
      "foo")

Whic means after deleting the import path, it changes to:

    import (
    )

In this case it still tries to merge the line with a non existing line,
causing token.File.MergeLine to panic.

We fix the issue by checking that the import path line is not the last
line to avoid panicing.

Fixes golang/go#20229

Change-Id: I37537a4eaa83d14db59a2926d7bb14c27167a2e4
Reviewed-on: https://go-review.googlesource.com/44372
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go/ast/astutil/imports.go

@@ -262,7 +262,7 @@ func DeleteNamedImport(fset *token.FileSet, f *ast.File, name, path string) (del
 					// There was a blank line immediately preceding the deleted import,
 					// so there's no need to close the hole.
 					// Do nothing.
-				} else {
+				} else if line != fset.File(gen.Rparen).LineCount() {
 					// There was no blank line. Close the hole.
 					fset.File(gen.Rparen).MergeLine(line)
 				}

go/ast/astutil/imports_test.go

@@ -1297,6 +1297,21 @@ import (
 	/* comment 2 */
 	"io"
 )
+`,
+	},
+
+	// Issue 20229: MergeLine panic on weird input
+	{
+		name: "import.37",
+		pkg:  "io",
+		in: `package main
+import("_"
+"io")`,
+		out: `package main
+
+import (
+	"_"
+)
 `,
 	},
 }