Limit the minimal size of bh_hashmap (#2073)

Limit the minimal size of bh_hashmap to avoid creating hashmap with size 0,
and `divide by zero` when calling bh_hash_map_find.

Reported by #2008.

core/shared/utils/bh_hashmap.h

@@ -12,6 +12,9 @@
 extern "C" {
 #endif
 
+/* Minimum initial size of hash map */
+#define HASH_MAP_MIN_SIZE 4
+
 /* Maximum initial size of hash map */
 #define HASH_MAP_MAX_SIZE 65536