Do not call freeaddrinfo(3) on nullptr

freeaddrinfo(3) only defined to free one or more addrinfo structures
returned by getaddrinfo(3).  Thus, it's undefined behaviour for calling
freeaddrinfo(3) on nullptr, some libc only call free(3) on the the
passed addrinfo, but other libc assumes the addrinfo is always valid.

Let's be explicit instead of relying on undefined behaviours.

Let's drop one call to freeaddrinfo(3) because the addrinfo there is
always nullptr and guard the second.

src/socket.cpp

@@ -108,7 +108,6 @@ class dap::Socket::Shared : public dap::ReaderWriter {
       return out;
     }
 
-    freeaddrinfo(info);
     term();
     return nullptr;
   }
@@ -117,7 +116,9 @@ class dap::Socket::Shared : public dap::ReaderWriter {
   Shared(addrinfo* info, SOCKET socket) : info(info), s(socket) {}
 
   ~Shared() {
+    if (info) {
         freeaddrinfo(info);
+    }
     close();
     term();
   }