cmd/go/internal/module: fix validation for module paths ending with /v

Unlike "/v1", "/v" is not likely to be mistaken for a semantic import path.

Change-Id: I024647d78c79c7761b98ddeccdc7e259ca94b568
Reviewed-on: https://go-review.googlesource.com/c/152738
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/module/module.go

@@ -284,7 +284,7 @@ func SplitPathVersion(path string) (prefix, pathMajor string, ok bool) {
 		}
 		i--
 	}
-	if i <= 1 || path[i-1] != 'v' || path[i-2] != '/' {
+	if i <= 1 || i == len(path) || path[i-1] != 'v' || path[i-2] != '/' {
 		return path, "", true
 	}
 	prefix, pathMajor = path[:i-2], path[i-2:]

src/cmd/go/internal/module/module_test.go

@@ -214,6 +214,7 @@ var splitPathVersionTests = []struct {
 	{"x.y/z", ""},
 	{"x.y/z", "/v2"},
 	{"x.y/z", "/v3"},
+	{"x.y/v", ""},
 	{"gopkg.in/yaml", ".v0"},
 	{"gopkg.in/yaml", ".v1"},
 	{"gopkg.in/yaml", ".v2"},