I would like to propose this minor change:

$ diff -wu lempar.c /usr/local/share/lemon/lempar.c 
--- lempar.c	2020-03-07 22:31:44.682974497 +0100
+++ /usr/local/share/lemon/lempar.c	2020-03-20 14:39:22.433113890 +0100
@@ -562,7 +562,7 @@
 #endif /* YYWILDCARD */
       return yy_default[stateno];
     }else{
-      assert( i>=0 && i<sizeof(yy_action)/sizeof(yy_action[0]) );
+      assert( i>=0 && (unsigned)i<sizeof(yy_action)/sizeof(yy_action[0]) );
       return yy_action[i];
     }
   }while(1);

This is semantically correct and quiets a warning when compiling with all warnings turned on.

Cheers!