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/gui.c b/src/gui.c
index 8568c20..8edc86c 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -2162,7 +2162,7 @@
 
     if (enc_utf8)
     {
-	buf = alloc((unsigned)(len * MB_MAXBYTES + 1));
+	buf = alloc(len * MB_MAXBYTES + 1);
 	if (buf == NULL)
 	    return OK; /* not much we could do here... */
 
@@ -2185,7 +2185,7 @@
     }
     else if (enc_dbcs == DBCS_JPNU)
     {
-	buf = alloc((unsigned)(len * 2 + 1));
+	buf = alloc(len * 2 + 1);
 	if (buf == NULL)
 	    return OK; /* not much we could do here... */