patch 8.2.3675: using freed memory when vim_strsave() fails

Problem:    Using freed memory when vim_strsave() fails.
Solution:   Clear "last_sourcing_name".  Check for msg_source() called
            recursively. (closes #8217)
diff --git a/src/message.c b/src/message.c
index e438211..8992de0 100644
--- a/src/message.c
+++ b/src/message.c
@@ -522,6 +522,12 @@
 msg_source(int attr)
 {
     char_u	*p;
+    static int	recursive = FALSE;
+
+    // Bail out if something called here causes an error.
+    if (recursive)
+	return;
+    recursive = TRUE;
 
     ++no_wait_return;
     p = get_emsg_source();
@@ -541,13 +547,13 @@
     // remember the last sourcing name printed, also when it's empty
     if (SOURCING_NAME == NULL || other_sourcing_name())
     {
-	vim_free(last_sourcing_name);
-	if (SOURCING_NAME == NULL)
-	    last_sourcing_name = NULL;
-	else
+	VIM_CLEAR(last_sourcing_name);
+	if (SOURCING_NAME != NULL)
 	    last_sourcing_name = vim_strsave(SOURCING_NAME);
     }
     --no_wait_return;
+
+    recursive = FALSE;
 }
 
 /*