patch 8.2.3939: MS-Windows: fnamemodify('', ':p') does not work

Problem:    MS-Windows: fnamemodify('', ':p') does not work.
Solution:   Do not consider an empty string a full path. (Yegappan Lakshmanan,
            closes #9428, closes #9427)
diff --git a/src/os_mswin.c b/src/os_mswin.c
index 26042b4..e27f389 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -390,7 +390,7 @@
     // the same as the name or mch_FullName() fails.  However, this has quite a
     // bit of overhead, so let's not do that.
     if (*fname == NUL)
-	return TRUE;
+	return FALSE;
     return ((ASCII_ISALPHA(fname[0]) && fname[1] == ':'
 				      && (fname[2] == '/' || fname[2] == '\\'))
 	    || (fname[0] == fname[1] && (fname[0] == '/' || fname[0] == '\\')));