patch 8.2.1842: crash when USE_FNAME_CASE is defined and using :browse

Problem:    Crash when USE_FNAME_CASE is defined and using :browse.
Solution:   Don't use read-only memory for ".". (Yegappan Lakshmanan,
            closes #7123)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index a55fae7..6371ece 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2465,6 +2465,7 @@
     bufref_T	old_curbuf;
     char_u	*free_fname = NULL;
 #ifdef FEAT_BROWSE
+    char_u	dot_path[] = ".";
     char_u	*browse_file = NULL;
 #endif
     int		retval = FAIL;
@@ -2511,7 +2512,7 @@
 		// No browsing supported but we do have the file explorer:
 		// Edit the directory.
 		if (ffname == NULL || !mch_isdir(ffname))
-		    ffname = (char_u *)".";
+		    ffname = dot_path;
 	    }
 	    else
 	    {