prevent panics when encountering nil diffs

While we can't change the behavior of helper/schema at this point, we
can protect against panics in the case of unexpected nils in the
instance diff.

builtin/providers/test/diff_apply_test.go

@@ -47,6 +47,8 @@ func TestDiffApply_set(t *testing.T) {
 			"egress.746197026.security_groups.#":   {Old: "", New: "0", NewComputed: false, NewRemoved: false},
 			"egress.746197026.self":                {Old: "", New: "false", NewComputed: false, NewRemoved: false},
 			"egress.746197026.to_port":             {Old: "", New: "8000", NewComputed: false, NewRemoved: false},
+			// an erroneous nil diff should do nothing
+			"egress.111111111.to_port": nil,
 		},
 	}
 

terraform/diff.go

@@ -513,6 +513,12 @@ func (d *InstanceDiff) applyBlockDiff(path []string, attrs map[string]string, sc
 		}
 
 		for k, diff := range d.Attributes {
+			// helper/schema should not insert nil diff values, but don't panic
+			// if it does.
+			if diff == nil {
+				continue
+			}
+
 			if strings.HasPrefix(k, blockKey) {
 				nextDot := strings.Index(k[len(blockKey):], ".")
 				if nextDot < 0 {
@@ -539,6 +545,12 @@ func (d *InstanceDiff) applyBlockDiff(path []string, attrs map[string]string, sc
 				// that we're dropping. Since we're only applying the "New"
 				// portion of the set, we can ignore diffs that only contain "Old"
 				for attr, diff := range d.Attributes {
+					// helper/schema should not insert nil diff values, but don't panic
+					// if it does.
+					if diff == nil {
+						continue
+					}
+
 					if !strings.HasPrefix(attr, indexPrefix) {
 						continue
 					}