Merge branch 'fix-wnon-literal-null-conversion-and-wconstant-conversion' into 'next'

Fix -Wnon-literal-null-conversion and -Wconstant-conversion warnings

See merge request STJr/SRB2!1929

src/p_spec.c

@@ -4396,7 +4396,7 @@ sector_t *P_FindPlayerTrigger(player_t *player, line_t *sourceline)
 			return loopsector;
 	}
 
-	return false;
+	return NULL;
 }
 
 boolean P_IsPlayerValid(size_t playernum)

src/taglist.c

@@ -472,5 +472,5 @@ mtag_t Tag_NextUnused(mtag_t start)
 		start++;
 	}
 
-	return MAXTAGS;
+	return (mtag_t)MAXTAGS;
 }