Avoid integer overflow. Addresses CVE-2015-2331.

Fixed similarly to patch used in PHP copy of libzip:
https://github.com/php/php-src/commit/ef8fc4b53d92fbfcd8ef1abbd6f2f5fe2c4a11e5

Thanks to Emmanuel Law <emmanuel.law@gmail.com> for the notification
about the bug.

lib/zip_dirent.c

diff --git a/lib/zip_dirent.c b/lib/zip_dirent.c
index 15ad6d7..ca1917c 100644
--- a/lib/zip_dirent.c
+++ b/lib/zip_dirent.c
@@ -105,7 +105,7 @@
 
     if (nentry == 0)
 	cd->entry = NULL;
-    else if ((cd->entry=(zip_entry_t *)malloc(sizeof(*(cd->entry))*(size_t)nentry)) == NULL) {
+    else if ((nentry > SIZE_MAX/sizeof(*(cd->entry))) || (cd->entry=(zip_entry_t *)malloc(sizeof(*(cd->entry))*(size_t)nentry)) == NULL) {
 	zip_error_set(error, ZIP_ER_MEMORY, 0);
 	free(cd);
 	return NULL;