setup: Fix bug with strdup() of NULL pointer.

getenv() can return NULL if the environment variable is not set, but
the result of getenv() was always being passed to M_StringDuplicate()
without any check. This could cause crashes on some platforms.
Instead, rework the code into a first stage that gets the player's
name and a second that duplicates it into a mutable form.

This fixes #455.

src/setup/multiplayer.c

@@ -1059,28 +1059,29 @@ void SetPlayerNameDefault(void)
 {
     if (net_player_name == NULL)
     {
-        net_player_name = M_StringDuplicate(getenv("USER"));
+        net_player_name = getenv("USER");
     }
 
     if (net_player_name == NULL)
     {
-        net_player_name = M_StringDuplicate(getenv("USERNAME"));
+        net_player_name = getenv("USERNAME");
     }
 
-    // On Windows, environment variables are in OEM codepage
-    // encoding, so convert to UTF8:
-
-#ifdef _WIN32
-    if (net_player_name != NULL)
-    {
-        net_player_name = M_OEMToUTF8(net_player_name);
-    }
-#endif
-
     if (net_player_name == NULL)
     {
-        net_player_name = M_StringDuplicate("player");
+        net_player_name = "player";
     }
+
+    // Now strdup() the string so that it's in a mutable form
+    // that can be freed when the value changes.
+
+#ifdef _WIN32
+    // On Windows, environment variables are in OEM codepage
+    // encoding, so convert to UTF8:
+    net_player_name = M_OEMToUTF8(net_player_name);
+#else
+    net_player_name = M_StringDuplicate(net_player_name);
+#endif
 }
 
 void MultiplayerConfig(void)