[PATCH] Fix crash in "document" command

[Tom Tromey <tromey@adacore.com>]

PR cli/29800 points out that "document" will now crash when the
argument is an undefined command.  This is a regression due to the
"document user-defined aliases" patch.
---
 gdb/cli/cli-script.c                | 2 ++
 gdb/testsuite/gdb.base/document.exp | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/gdb/cli/cli-script.c b/gdb/cli/cli-script.c
index 2442f7ae38b..2101d6fface 100644
--- a/gdb/cli/cli-script.c
+++ b/gdb/cli/cli-script.c
@@ -1515,6 +1515,8 @@ do_document_command (const char *comname, int from_tty,
   validate_comname (&comname);
 
   lookup_cmd_composition (comfull, &alias, &prefix_cmd, &c);
+  if (c == nullptr)
+    error (_("Undefined command: \"%s\"."), comfull);
 
   if (c->theclass != class_user
       && (alias == nullptr || alias->theclass != class_alias))
diff --git a/gdb/testsuite/gdb.base/document.exp b/gdb/testsuite/gdb.base/document.exp
index 2d8f6b88090..6f426510d29 100644
--- a/gdb/testsuite/gdb.base/document.exp
+++ b/gdb/testsuite/gdb.base/document.exp
@@ -28,3 +28,7 @@ gdb_test_multiple "define do-document" "" {
 }
 gdb_test_no_output "do-document" "invoke do-document"
 gdb_test "help do-document" "usage: do-document" "invoke help do-document"
+
+# Test that document of a non-existing command prints an error.  There
+# was a regression at one point causing this to crash.
+gdb_test "document nosuchcommand" "Undefined command: \"nosuchcommand\"\\."
-- 
2.34.3