Allow NPCs to accept exact trades. (CleverRaven#49470)

This solves issues where an NPC offers to train you for (say) $10, but
actually insists on $10.01 or more in value.

src/npctrade.cpp

@@ -754,5 +754,5 @@ bool npc_trading::trade( npc &np, int cost, const std::string &deal )
 // Will the NPC accept the trade that's currently on offer?
 bool trading_window::npc_will_accept_trade( const npc &np ) const
 {
-    return np.will_exchange_items_freely() || your_balance + np.max_credit_extended() > 0;
+    return np.will_exchange_items_freely() || your_balance + np.max_credit_extended() >= 0;
 }