patch 8.1.0976: dosinstall still has buffer overflow problems

Problem:    Dosinstall still has buffer overflow problems.
Solution:   Adjust  buffer sizes. (Yasuhiro Matsumoto, closes #4002)
diff --git a/src/dosinst.h b/src/dosinst.h
index f264f94..7f30e90 100644
--- a/src/dosinst.h
+++ b/src/dosinst.h
@@ -59,7 +59,7 @@
 /* ---------------------------------------- */
 
 
-#define BUFSIZE 512		/* long enough to hold a file name path */
+#define BUFSIZE (MAX_PATH*2)		/* long enough to hold a file name path */
 #define NUL 0
 
 #define FAIL 0
@@ -93,15 +93,15 @@
     static void *
 alloc(int len)
 {
-    char *s;
+    void *p;
 
-    s = malloc(len);
-    if (s == NULL)
+    p = malloc(len);
+    if (p == NULL)
     {
 	printf("ERROR: out of memory\n");
 	exit(1);
     }
-    return (void *)s;
+    return p;
 }
 
 /*
@@ -512,7 +512,7 @@
 do_inits(char **argv)
 {
     /* Find out the full path of our executable. */
-    if (my_fullpath(installdir, argv[0], BUFSIZE) == NULL)
+    if (my_fullpath(installdir, argv[0], sizeof(installdir)) == NULL)
     {
 	printf("ERROR: Cannot get name of executable\n");
 	myexit(1);