Fix valgrind warnings and spurious error messages

Just clean up valgrind warnings about uninitialized memory
and also clear out errno in some cases where it results in
a false error message being generated at a later point.

src/checkout.c

@@ -31,7 +31,7 @@ typedef struct tree_walk_data
 	git_checkout_opts *opts;
 	git_repository *repo;
 	git_odb *odb;
-	bool do_symlinks;
+	bool no_symlinks;
 } tree_walk_data;
 
 
@@ -48,9 +48,9 @@ static int blob_contents_to_link(tree_walk_data *data, git_buf *fnbuf,
 			/* Create the link */
 			const char *new = git_buf_cstr(&linktarget),
 						  *old = git_buf_cstr(fnbuf);
-			retcode = data->do_symlinks
-				? p_symlink(new, old)
-				: git_futils_fake_symlink(new, old);
+			retcode = data->no_symlinks
+				? git_futils_fake_symlink(new, old)
+				: p_symlink(new, old);
 		}
 		git_buf_free(&linktarget);
 		git_blob_free(blob);
@@ -176,13 +176,14 @@ int git_checkout_head(git_repository *repo, git_checkout_opts *opts, git_indexer
 		return GIT_ERROR;
 	}
 
+	memset(&payload, 0, sizeof(payload));
+
 	/* Determine if symlinks should be handled */
-	if (!git_repository_config(&cfg, repo)) {
+	if (!git_repository_config__weakptr(&cfg, repo)) {
 		int temp = true;
 		if (!git_config_get_bool(&temp, cfg, "core.symlinks")) {
-			payload.do_symlinks = !!temp;
+			payload.no_symlinks = !temp;
 		}
-		git_config_free(cfg);
 	}
 
 	stats->total = stats->processed = 0;

src/errors.c

@@ -110,6 +110,11 @@ void giterr_set_regex(const regex_t *regex, int error_code)
 void giterr_clear(void)
 {
 	GIT_GLOBAL->last_error = NULL;
+
+	errno = 0;
+#ifdef GIT_WIN32
+	SetLastError(0);
+#endif
 }
 
 const git_error *giterr_last(void)

src/filebuf.c

@@ -50,6 +50,7 @@ static int lock_file(git_filebuf *file, int flags)
 		if (flags & GIT_FILEBUF_FORCE)
 			p_unlink(file->path_lock);
 		else {
+			giterr_clear(); /* actual OS error code just confuses */
 			giterr_set(GITERR_OS,
 				"Failed to lock file '%s' for writing", file->path_lock);
 			return -1;

src/submodule.c

@@ -367,6 +367,8 @@ int git_submodule_add_to_index(git_submodule *sm, int write_index)
 		error = -1;
 		goto cleanup;
 	}
+
+	memset(&entry, 0, sizeof(entry));
 	entry.path = sm->path;
 	git_index__init_entry_from_stat(&st, &entry);