go/build: correct shouldBuild bug reading whole contents of file.

It was caused by bytes.TrimSpace being able to return a nil
slice.

Fixes #3914.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/6458091

src/pkg/go/build/build.go

@@ -689,7 +689,7 @@ func (ctxt *Context) shouldBuild(content []byte) bool {
 		}
 		line = bytes.TrimSpace(line)
 		if len(line) == 0 { // Blank line
-			end = cap(content) - cap(line) // &line[0] - &content[0]
+			end = len(content) - len(p)
 			continue
 		}
 		if !bytes.HasPrefix(line, slashslash) { // Not comment line

src/pkg/go/build/build_test.go

@@ -75,3 +75,32 @@ func TestLocalDirectory(t *testing.T) {
 		t.Fatalf("ImportPath=%q, want %q", p.ImportPath, "go/build")
 	}
 }
+
+func TestShouldBuild(t *testing.T) {
+	const file1 = "// +build tag1\n\n" +
+		"package main\n"
+
+	const file2 = "// +build cgo\n\n" +
+		"// This package implements parsing of tags like\n" +
+		"// +build tag1\n" +
+		"package build"
+
+	const file3 = "// Copyright The Go Authors.\n\n" +
+		"package build\n\n" +
+		"// shouldBuild checks tags given by lines of the form\n" +
+		"// +build tag\n" +
+		"func shouldBuild(content []byte)\n"
+
+	ctx := &Context{BuildTags: []string{"tag1"}}
+	if !ctx.shouldBuild([]byte(file1)) {
+		t.Errorf("should not build file1, expected the contrary")
+	}
+	if ctx.shouldBuild([]byte(file2)) {
+		t.Errorf("should build file2, expected the contrary")
+	}
+
+	ctx = &Context{BuildTags: nil}
+	if !ctx.shouldBuild([]byte(file3)) {
+		t.Errorf("should not build file3, expected the contrary")
+	}
+}