Disallow changing a vein into itself

Prevents infinite recursion from occurring while searching for
neighboring vein tiles, which would eventually result in a crash.

Fixes DFHack#5231

Author: NicksWorld

docs/changelog.txt

@@ -61,6 +61,7 @@ Template for new versions:
 
 ## Fixes
 - `spectate`: don't allow temporarily modified announcement settings to be written to disk when "auto-unpause" mode is enabled
+- `changevein`: fix a crash that could occur when attempting to change a vein into itself
 
 ## Misc Improvements
 - `spectate`: player-set configuration is now stored globally instead of per-fort

plugins/changevein.cpp

@@ -254,6 +254,12 @@ command_result df_changevein (color_ostream &out, vector <string> & parameters)
         return CR_FAILURE;
     }
 
+    if (mineral->inorganic_mat == mi.index)
+    {
+        out.printerr("Selected tile is already of the target material.\n");
+        return CR_FAILURE;
+    }
+
     VeinEdgeBitmask mask = VeinEdgeBitmask(mineral);
     mineral->inorganic_mat = mi.index;
     ChangeSameBlockVeins(block, mineral, mask, mi.index);