don't panic on nil field during inner join

when running a select statement with an inner join where the secondary field is non-existent, we get a panic. this commit fixes that.

see [fb-766](https://molecula.atlassian.net/browse/FB-766) for more information.

server/grpc_test.go

@@ -1072,6 +1072,10 @@ func TestQuerySQLWithError(t *testing.T) {
 			sql: "select age, color, count(*) from grouper group by field_not_found, age, color",
 			err: pilosa.ErrFieldNotFound,
 		},
+		{
+			sql: "select count(*) from grouper inner join joiner on grouper._id = joiner.field_not_found",
+			err: pilosa.ErrFieldNotFound,
+		},
 	}
 
 	for i, test := range tests {

sql/select.go

@@ -772,6 +772,9 @@ func (h handlerSelectJoin) Apply(stmt *sqlparser.Select, qm QueryMask, indexFunc
 		return nil, fmt.Errorf("nonexistent index %q", secondaryIndexName)
 	}
 	secondaryField := secondaryIndex.Field(secondary.column.name)
+	if secondaryField == nil {
+		return nil, errors.Wrap(pilosa.ErrFieldNotFound, fmt.Sprintf("non-existent field %s for index %s", secondary.column.name, secondaryIndexName))
+	}
 
 	var wheres tableWheres
 	if qm.HasWhere() {