Allow to skip malleability check when deserializing BLS primitives

Useful when many sigs need to be deserialized and at the same time the hash
of these is never used.

src/bls/bls.h

@@ -165,13 +165,15 @@ public:
         // }
     }
     template <typename Stream>
-    inline void Unserialize(Stream& s)
+    inline void Unserialize(Stream& s, bool checkMalleable = true)
     {
         char buf[SerSize];
         s.read((char*)buf, SerSize);
         SetBuf(buf, SerSize);
 
-        CheckMalleable(buf, SerSize);
+        if (checkMalleable) {
+            CheckMalleable(buf, SerSize);
+        }
     }
 
     inline void CheckMalleable(void* buf, size_t size) const