fix abstract nondet exception

test/libsolidity/smtCheckerTests/natspec/abstract_free_function_1.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.8.13;
+
+contract C {
+    function f(uint x) external pure {
+        uint t = msb(x);
+        assert(t == 0); // should fail
+    }
+}
+
+/// @custom:smtchecker abstract-function-nondet
+function msb(uint256 x) pure returns (uint256 result) {}
+
+// ====
+// SMTEngine: chc
+// ----
+// Warning 6328: (144-158): CHC: Assertion violation happens here.