Remove multiple calls to free when successively calling jq_reset.

`jq_reset` calls `jv_free` on the `exit_code` and the `error_message` stored on the jq state.
However, it doesn't replace the actual instance of those members. This means that subsequent
calls to `jq_reset` will call `jv_free` again on those members, which in turn may call `free`
on the same pointer multiple times. Freeing the same pointer multiple times is undefined
behavior and can cause heap corruption, which is how I spotted this issue.

In practice, this issue only occurs when using a program that may `halt_error`, because that
is when the `exit_code` and `error_message` are set to values other than `jv_invalid`.
Subsequent attempts to call `jq_start` (which calls `jq_reset` internally) after hitting a
`halt_error` can cause you to run into this issue.

The changes simply reset the `exit_code` and the `error_message` to `jv_invalid` (the initial
value set in `jq_init`) after they are freed.

src/execute.c

@@ -315,7 +315,9 @@ static void jq_reset(jq_state *jq) {
 
   jq->halted = 0;
   jv_free(jq->exit_code);
+  jq->exit_code = jv_invalid();
   jv_free(jq->error_message);
+  jq->error_message = jv_invalid();
   if (jv_get_kind(jq->path) != JV_KIND_INVALID)
     jv_free(jq->path);
   jq->path = jv_null();

src/jq_test.c

@@ -10,6 +10,7 @@
 
 static void jv_test();
 static void run_jq_tests(jv, int, FILE *, int, int);
+static void run_jq_start_state_tests();
 #ifdef HAVE_PTHREAD
 static void run_jq_pthread_tests();
 #endif
@@ -37,6 +38,7 @@ int jq_testsuite(jv libdirs, int verbose, int argc, char* argv[]) {
     }
   }
   run_jq_tests(libdirs, verbose, testdata, skip, take);
+  run_jq_start_state_tests();
 #ifdef HAVE_PTHREAD
   run_jq_pthread_tests();
 #endif
@@ -251,6 +253,66 @@ static void run_jq_tests(jv lib_dirs, int verbose, FILE *testdata, int skip, int
 }
 
 
+static int test_start_state(jq_state *jq, char *prog) {
+  int pass = 1;
+  jv message = jq_get_error_message(jq);
+  if (jv_is_valid(message)) {
+    printf("*** Expected error_message to be invalid after jq_start: %s\n", prog);
+    pass = 0;
+  }
+  jv_free(message);
+
+  jv exit_code = jq_get_exit_code(jq);
+  if (jv_is_valid(exit_code)) {
+    printf("*** Expected exit_code to be invalid after jq_start: %s\n", prog);
+    pass = 0;
+  }
+  jv_free(exit_code);
+
+  if (jq_halted(jq)) {
+    printf("*** Expected jq to not be halted after jq_start: %s\n", prog);
+    pass = 0;
+  }
+
+  return pass;
+}
+
+// Test jq_state is reset after subsequent calls to jq_start.
+static void test_jq_start_resets_state(char *prog, const char *input) {
+  printf("Test jq_state: %s\n", prog);
+  jq_state *jq = jq_init();
+  assert(jq);
+
+  int compiled = jq_compile(jq, prog);
+  assert(compiled);
+
+  // First call to jq_start. Run until completion.
+  jv parsed_input = jv_parse(input);
+  assert(jv_is_valid(parsed_input));
+  jq_start(jq, parsed_input, 0);
+  assert(test_start_state(jq, prog));
+  while (1) {
+    jv result = jq_next(jq);
+    int valid = jv_is_valid(result);
+    jv_free(result);
+    if (!valid) { break; }
+  }
+
+  // Second call to jq_start.
+  jv parsed_input2 = jv_parse(input);
+  assert(jv_is_valid(parsed_input2));
+  jq_start(jq, parsed_input2, 0);
+  assert(test_start_state(jq, prog));
+
+  jq_teardown(&jq);
+}
+
+static void run_jq_start_state_tests() {
+  test_jq_start_resets_state(".[]", "[1,2,3]");
+  test_jq_start_resets_state(".[] | if .%2 == 0 then halt_error else . end", "[1,2,3]");
+}
+
+
 /// pthread regression test
 #ifdef HAVE_PTHREAD
 #define NUMBER_OF_THREADS 3