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_win32.c b/src/os_win32.c
index 237de97..d290710 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -2075,8 +2075,7 @@
 	return FALSE;
 
     wcurpath = _wgetenv(L"PATH");
-    wnewpath = (WCHAR*)alloc((unsigned)(wcslen(wcurpath) + 3)
-	    * sizeof(WCHAR));
+    wnewpath = (WCHAR *)alloc((wcslen(wcurpath) + 3) * sizeof(WCHAR));
     if (wnewpath == NULL)
 	return FALSE;
     wcscpy(wnewpath, L".;");
@@ -7205,7 +7204,7 @@
     char_u	*envbuf;
     WCHAR	*p;
 
-    envbuf = alloc((unsigned)(STRLEN(var) + STRLEN(value) + 2));
+    envbuf = alloc(STRLEN(var) + STRLEN(value) + 2);
     if (envbuf == NULL)
 	return -1;