modules: Add newlen == 0 handling to RM_StringTruncate (#3717)

Previously, passing 0 for newlen would not truncate the string at all.

This adds handling of this case, freeing the old string and creating a new empty string.

src/module.c

@@ -1446,14 +1446,19 @@ int RM_StringTruncate(RedisModuleKey *key, size_t newlen) {
     } else {
         /* Unshare and resize. */
         key->value = dbUnshareStringValue(key->db, key->key, key->value);
-        size_t curlen = sdslen(key->value->ptr);
-        if (newlen > curlen) {
-            key->value->ptr = sdsgrowzero(key->value->ptr,newlen);
-        } else if (newlen < curlen) {
-            sdsrange(key->value->ptr,0,newlen-1);
-            /* If the string is too wasteful, reallocate it. */
-            if (sdslen(key->value->ptr) < sdsavail(key->value->ptr))
-                key->value->ptr = sdsRemoveFreeSpace(key->value->ptr);
+	if (newlen == 0) {
+	    sdsfree(key->value->ptr);
+	    key->value->ptr = sdsempty();
+	} else {
+	    size_t curlen = sdslen(key->value->ptr);
+	    if (newlen > curlen) {
+                key->value->ptr = sdsgrowzero(key->value->ptr,newlen);
+	    } else if (newlen < curlen) {
+                sdsrange(key->value->ptr,0,newlen-1);
+                /* If the string is too wasteful, reallocate it. */
+                if (sdslen(key->value->ptr) < sdsavail(key->value->ptr))
+	            key->value->ptr = sdsRemoveFreeSpace(key->value->ptr);
+	    }
         }
     }
     return REDISMODULE_OK;