Corrected error in technology file reloading that causes the plow

module to fill up its rules table and start printing errors about
the table overflow.  Tables are now properly reset on tech reload.

plow/PlowMain.c

@@ -2051,6 +2051,10 @@ PlowAfterTech()
     TileTypeBitMask mask;
     TileType i, j;
 
+    /* Reset rules tables */
+    plowSearchRulesPtr = plowSearchRulesTbl;
+    plowCellRulesPtr = plowCellRulesTbl;
+
     /* Set the masks we will use for all the rules below */
     allButSpace = DBAllButSpaceAndDRCBits;
     allBits = DBAllTypeBits;