Add some casts for gcc 4.8.3 with more-than-default warnings enabled.

lib/zip_string.c

diff --git a/lib/zip_string.c b/lib/zip_string.c
index a731ebb..d00e3aa 100644
--- a/lib/zip_string.c
+++ b/lib/zip_string.c
@@ -154,7 +154,7 @@
 	return NULL;
     }
 
-    if ((s->raw=(zip_uint8_t *)malloc(length+1)) == NULL) {
+    if ((s->raw=(zip_uint8_t *)malloc((size_t)(length+1))) == NULL) {
 	free(s);
 	return NULL;
     }