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.

src/charset.c

@@ -355,10 +355,10 @@ transstr(char_u *s)
 		    len += 4;	/* illegal byte sequence */
 	    }
 	}
-	res = alloc((unsigned)(len + 1));
+	res = alloc(len + 1);
     }
     else
-	res = alloc((unsigned)(vim_strsize(s) + 1));
+	res = alloc(vim_strsize(s) + 1);
     if (res != NULL)
     {
 	*res = NUL;