fix: part (#1599)

## Description

minor fix

Author: evan-forbes

types/part_set.go

@@ -37,6 +37,9 @@ func (part *Part) ValidateBasic() error {
 	if int64(part.Index) < part.Proof.Total-1 && len(part.Bytes) != int(BlockPartSizeBytes) {
 		return ErrPartInvalidSize
 	}
+	if int64(part.Index) != part.Proof.Index {
+		return fmt.Errorf("part index %d != proof index %d", part.Index, part.Proof.Index)
+	}
 	if err := part.Proof.ValidateBasic(); err != nil {
 		return fmt.Errorf("wrong Proof: %w", err)
 	}

types/part_set_test.go

@@ -137,19 +137,27 @@ func TestPartValidateBasic(t *testing.T) {
 			pt.Index = 1
 			pt.Bytes = make([]byte, BlockPartSizeBytes-1)
 			pt.Proof.Total = 2
+			pt.Proof.Index = 1
 		}, false},
 		{"Too small inner part", func(pt *Part) {
 			pt.Index = 0
 			pt.Bytes = make([]byte, BlockPartSizeBytes-1)
+			pt.Proof.Index = 1
 			pt.Proof.Total = 2
-		}, true},
+		},
+
+			true},
 		{"Too big proof", func(pt *Part) {
 			pt.Proof = merkle.Proof{
 				Total:    2,
 				Index:    1,
 				LeafHash: make([]byte, 1024*1024),
 			}
 		}, true},
+		{"Index mismatch", func(pt *Part) {
+			pt.Index = 1
+			pt.Proof.Index = 0
+		}, true},
 	}
 
 	for _, tc := range testCases {