Make can_trade() return true if at least one caravan can trade

Fixes #900

NEWS.rst

@@ -84,6 +84,7 @@ Fixes
 - More plugins should recognize non-dwarf citizens
 - Fixed a possible crash from cloning jobs
 - moveToBuilding() now sets flags for items that aren't a structural part of the building properly
+- `autotrade`, `stocks`: Made trading work when multiple caravans are present but only some can trade
 - `confirm` note-delete: No longer interferes with name entry
 - `exportlegends`: Handles entities without specific races, and a few other fixes for things new to v0.42
 - `fastdwarf`: Fixed a bug involving teleporting mothers but not the babies they're holding.

plugins/uicommon.h

@@ -263,11 +263,11 @@ static bool can_trade()
         auto caravan = *it;
         auto trade_state = caravan->trade_state;
         auto time_remaining = caravan->time_remaining;
-        if ((trade_state != state::Approaching && trade_state != state::AtDepot) || time_remaining == 0)
-            return false;
+        if ((trade_state == state::Approaching || trade_state == state::AtDepot) && time_remaining != 0)
+            return true;
     }
 
-    return true;
+    return false;
 }
 
 static bool is_metal_item(df::item *item)