Rust: Fix bug when there are no statements.

Author: geoffw0

rust/ql/lib/codeql/rust/elements/internal/StmtListImpl.qll

@@ -33,7 +33,7 @@ module Impl {
     AstNode getStmtOrExpr(int index) {
       result = this.getStatement(index)
       or
-      index = max(int i | exists(this.getStatement(i))) + 1 and
+      index = max(int i | i = -1 or exists(this.getStatement(i))) + 1 and
       result = this.getTailExpr()
     }
 

rust/ql/test/library-tests/elements/stmtlist/StmtList.expected

@@ -1,9 +1,9 @@
 | StmtList.rs:4:19:9:1 | StmtList | 2 | hasTailExpr | 0:let ... = 1, 1:let ... = 2, 2:... + ... |
 | StmtList.rs:11:18:15:1 | StmtList | 2 |  | 0:let ... = 1, 1:let ... = 2 |
-| StmtList.rs:17:19:20:1 | StmtList | 0 | hasTailExpr |  |
+| StmtList.rs:17:19:20:1 | StmtList | 0 | hasTailExpr | 0:... + ... |
 | StmtList.rs:22:18:25:1 | StmtList | 1 |  | 0:ExprStmt |
 | StmtList.rs:27:18:29:1 | StmtList | 0 |  |  |
 | StmtList.rs:31:18:34:1 | StmtList | 0 |  |  |
-| StmtList.rs:36:29:43:1 | StmtList | 0 | hasTailExpr |  |
-| StmtList.rs:38:10:40:2 | StmtList | 0 | hasTailExpr |  |
-| StmtList.rs:40:9:42:2 | StmtList | 0 | hasTailExpr |  |
+| StmtList.rs:36:29:43:1 | StmtList | 0 | hasTailExpr | 0:if cond {...} else {...} |
+| StmtList.rs:38:10:40:2 | StmtList | 0 | hasTailExpr | 0:1 |
+| StmtList.rs:40:9:42:2 | StmtList | 0 | hasTailExpr | 0:2 |