global: free the error message when exiting a thread

When we free the global state at thread termination, we must also free
the error message in order not to leak the string once per thread.

src/global.c

@@ -223,6 +223,9 @@ int init_error = 0;
 
 static void cb__free_status(void *st)
 {
+	git_global_st *state = (git_global_st *) st;
+	git__free(state->error_t.message);
+
 	git__free(st);
 }
 

tests/threads/basic.c

 #include "clar_libgit2.h"
 
+#include "thread_helpers.h"
 #include "cache.h"
 
 
@@ -34,3 +35,16 @@ void test_threads_basic__multiple_init(void)
 	cl_git_pass(git_repository_open(&nested_repo, cl_fixture("testrepo.git")));
 	git_repository_free(nested_repo);
 }
+
+static void *set_error(void *dummy)
+{
+	giterr_set(GITERR_INVALID, "oh no, something happened!\n");
+
+	return dummy;
+}
+
+/* Set errors so we can check that we free it */
+void test_threads_basic__set_error(void)
+{
+	run_in_parallel(1, 4, set_error, NULL, NULL);
+}