floor difficulty_mult at 0.1 for bad wp

src/weakpoint.cpp

@@ -552,7 +552,7 @@ float weakpoint::hit_chance( const weakpoint_attack &attack ) const
     } else {
         // Use erfc if the wp does not benefit the attacker.
         diff = attack.wp_skill - std::max( difficulty.of( attack ) + 10.0f, 10.0f );
-        difficulty_mult = 0.5f * erfc( diff / ( 2.0f * sqrt( 2.0f ) ) );
+        difficulty_mult = std::max( 0.5f * erfc( diff / ( 2.0f * sqrt( 2.0f ) ) ), 0.1f );
         final_coverage = coverage;
     }