Fix -trace-save crash when argument is missing

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.