prefix_filename(): safely handle the case where pfx_len=0

Current prefix_filename() is proofed against the case where the prefix
'pfx' is NULL or a 0-length string, _except on Windows_.

Change the behaviour to work the same on both platforms, and only
check pfx_len so that callers passing a NULL prefix with a nonzero
pfx_len segfault early on both.
Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -46,7 +46,7 @@ const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
 {
 	static char path[PATH_MAX];
 #ifndef WIN32
-	if (!pfx || !*pfx || is_absolute_path(arg))
+	if (!pfx_len || is_absolute_path(arg))
 		return arg;
 	memcpy(path, pfx, pfx_len);
 	strcpy(path + pfx_len, arg);
@@ -55,7 +55,7 @@ const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
 	/* don't add prefix to absolute paths, but still replace '\' by '/' */
 	if (is_absolute_path(arg))
 		pfx_len = 0;
-	else
+	else if (pfx_len)
 		memcpy(path, pfx, pfx_len);
 	strcpy(path + pfx_len, arg);
 	for (p = path + pfx_len; *p; p++)