Return an error if object keys are not strings

This now gives an error instead of a panic when encountering
configuration such as described in hashicorp/terraform#5740:

```
resource "aws" "web" {
  provider = "aws" {
    region = "us-west-2"
  }
}
```

We now return an error message - "hcl object keys must be a string"
instead of crashing.

Fixes hashicorp/terraform#5740.

decoder.go

@@ -337,6 +337,14 @@ func (d *decoder) decodeMap(name string, node ast.Node, result reflect.Value) er
 			continue
 		}
 
+		// github.com/hashicorp/terraform/issue/5740
+		if len(item.Keys) == 0 {
+			return &parser.PosError{
+				Pos: node.Pos(),
+				Err: fmt.Errorf("%s: map must have string keys", name),
+			}
+		}
+
 		// Get the key we're dealing with, which is the first item
 		keyStr := item.Keys[0].Token.Value().(string)
 

decoder_test.go

@@ -245,6 +245,14 @@ func TestDecode_interface(t *testing.T) {
 			nil,
 		},
 
+		{
+			"nested_provider_bad.hcl",
+			true,
+			// This is not ideal but without significant rework of the decoder
+			// we get a partial result back as well as an error.
+			map[string]interface{}{},
+		},
+
 		{
 			"object_list.json",
 			false,

test-fixtures/nested_provider_bad.hcl

@@ -0,0 +1,5 @@
+resource "aws" "web" {
+  provider = "aws" {
+    region = "us-west-2"
+  }
+}