fix crash when build.sources is set to an invalid name

the `build` body doesn't have any labels and we were trying to display those.
I also added a test.

hcl2template/testdata/build/invalid_source_reference.pkr.hcl

@@ -0,0 +1,4 @@
+
+build {
+  sources = ["ami"]
+}

hcl2template/types.build.go

@@ -51,11 +51,11 @@ func (p *Parser) decodeBuildConfig(block *hcl.Block) (*BuildBlock, hcl.Diagnosti
 		if ref == NoSource {
 			diags = append(diags, &hcl.Diagnostic{
 				Severity: hcl.DiagError,
-				Summary:  "Invalid " + sourceLabel + " reference",
+				Summary:  "Invalid " + sourceLabel + " reference: " + buildFrom,
 				Detail: "A " + sourceLabel + " type must start with a letter and " +
 					"may contain only letters, digits, underscores, and dashes." +
 					"A valid source reference looks like: `source.type.name`",
-				Subject: &block.LabelRanges[0],
+				Subject: block.DefRange.Ptr(),
 			})
 			continue
 		}
@@ -67,7 +67,7 @@ func (p *Parser) decodeBuildConfig(block *hcl.Block) (*BuildBlock, hcl.Diagnosti
 				Detail: "A " + sourceLabel + " type must start with a letter and " +
 					"may contain only letters, digits, underscores, and dashes." +
 					"A valid source reference looks like: `source.type.name`",
-				Subject: &block.LabelRanges[0],
+				Subject: block.DefRange.Ptr(),
 			})
 			continue
 		}
@@ -75,7 +75,8 @@ func (p *Parser) decodeBuildConfig(block *hcl.Block) (*BuildBlock, hcl.Diagnosti
 		build.Froms = append(build.Froms, ref)
 	}
 
-	content, diags := b.Config.Content(buildSchema)
+	content, moreDiags := b.Config.Content(buildSchema)
+	diags = append(diags, moreDiags...)
 	for _, block := range content.Blocks {
 		switch block.Type {
 		case buildProvisionerLabel:

hcl2template/types.build_test.go

@@ -83,6 +83,16 @@ func TestParse_build(t *testing.T) {
 			[]packer.Build{},
 			false,
 		},
+		{"invalid source",
+			defaultParser,
+			parseTestArgs{"testdata/build/invalid_source_reference.pkr.hcl"},
+			&PackerConfig{
+				Builds: nil,
+			},
+			true, true,
+			[]packer.Build{},
+			false,
+		},
 	}
 	testParse(t, tests)
 }