fix possible out of range error in Prefix ctor

include/opendht/indexation/pht.h

@@ -28,10 +28,13 @@ struct Prefix {
     Prefix() {}
     Prefix(InfoHash h) : size_(h.size() * 8), content_(h.begin(), h.end()) {}
     Prefix(const Blob& d) : size_(d.size()*8), content_(d) {}
-    Prefix(const Prefix& p, size_t first) : size_(first), content_(p.content_.begin(), p.content_.begin()+first/8) {
-       auto rem = first % 8;
+    Prefix(const Prefix& p, size_t first) :
+        size_(std::min(first, p.content_.size()*8)),
+        content_(Blob(p.content_.begin(), p.content_.begin()+size_/8))
+    {
+        auto rem = size_ % 8;
         if (rem)
-           content_.push_back(p.content_[first/8] & (0xFF << (7 - rem)));
+            content_.push_back(p.content_[size_/8] & (0xFF << (7 - rem)));
     }
 
     Prefix getPrefix(ssize_t len) const {