patch 8.1.1384: using "int" for alloc() often results in compiler warnings

Problem:    Using "int" for alloc() often results in compiler warnings.
Solution:   Use "size_t" and remove type casts.  Remove alloc_check(), Vim
            only works with 32 bit ints anyway.
diff --git a/src/menu.c b/src/menu.c
index 6e0c02e..1ff45b9 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -694,7 +694,7 @@
 		 * \'s and ^V's stripped out. But menu_path is a "raw"
 		 * string, so we must correct for special characters.
 		 */
-		tearpath = alloc((unsigned int)STRLEN(menu_path) + TEAR_LEN + 2);
+		tearpath = alloc(STRLEN(menu_path) + TEAR_LEN + 2);
 		if (tearpath != NULL)
 		{
 		    char_u  *s;
@@ -780,7 +780,7 @@
 
 		if (c != 0)
 		{
-		    menu->strings[i] = alloc((unsigned)(STRLEN(call_data) + 5 ));
+		    menu->strings[i] = alloc(STRLEN(call_data) + 5);
 		    if (menu->strings[i] != NULL)
 		    {
 			menu->strings[i][0] = c;
@@ -1316,7 +1316,7 @@
 	menu = root_menu;
 	if (after_dot != arg)
 	{
-	    path_name = alloc((unsigned)(after_dot - arg));
+	    path_name = alloc(after_dot - arg);
 	    if (path_name == NULL)
 		return NULL;
 	    vim_strncpy(path_name, arg, after_dot - arg - 1);