Fixed possible buffer overflow bug if RDB file is corrupted.

(Note: commit message modified by @antirez for clarity).

src/rdb.c

@@ -410,7 +410,7 @@ int rdbSaveDoubleValue(rio *rdb, double val) {
 
 /* For information about double serialization check rdbSaveDoubleValue() */
 int rdbLoadDoubleValue(rio *rdb, double *val) {
-    char buf[128];
+    char buf[256];
     unsigned char len;
 
     if (rioRead(rdb,&len,1) == 0) return -1;