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_amiga.c b/src/os_amiga.c
index 4a6814c..ec392cd 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -1467,7 +1467,7 @@
     {
 #endif
 	/* hack to replace '*' by '#?' */
-	starbuf = alloc((unsigned)(2 * STRLEN(pat) + 1));
+	starbuf = alloc(2 * STRLEN(pat) + 1);
 	if (starbuf == NULL)
 	    goto Return;
 	for (sp = pat, dp = starbuf; *sp; ++sp)