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/os_vms.c b/src/os_vms.c
index 61db3a8..ca04019 100644
--- a/src/os_vms.c
+++ b/src/os_vms.c
@@ -238,7 +238,7 @@
     if (sys$trnlnm(&attrib, &d_file_dev, &d_lognam, NULL,&itmlst) == SS$_NORMAL)
     {
 	buffer[lengte] = '\0';
-	if (cp = (char_u *)alloc((unsigned)(lengte+1)))
+	if (cp = (char_u *)alloc(lengte + 1))
 	    strcpy((char *)cp, buffer);
 	return(cp);
     }