Fix -trace-save crash when argument is missing

-trace-save doesn't check whether an argument is passed, leading to a
segfault if you pass nothing.

I added a small test, which only tests the error conditions of
-trace-save.

gdb/ChangeLog:

	* mi/mi-main.c (mi_cmd_trace_save): Check if argument is present
	before using it.

gdb/testsuite/ChangeLog:

	* gdb.trace/mi-trace-save.exp: New file.

1 files changed, 5 insertions, 0 deletions

diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index 7cd9706..81e82ed 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -2699,6 +2699,11 @@ mi_cmd_trace_save (char *command, char **argv, int argc)
 	  break;
 	}
     }
+
+  if (argc - oind != 1)
+    error (_("Exactly one argument required "
+	     "(file in which to save trace data)"));
+
   filename = argv[oind];
 
   if (generate_ctf)