infcmd.c: Don't attempt to record a NULL value after a finish command.

infcmd.c: Don't attempt to record a NULL value after a finish command.

Architectures which use RETURN_VALUE_STRUCT_CONVENTION will have a
NULL return value after executing a finish command.  See get_return_value()
in infcmd.c.

This patch avoids an eventual SIGSEV (caused by attempting to
derefrence a NULL pointer) by adding a suitable test to
finish_command_fsm_should_stop().

I encountered this problem while testing msp430:

(gdb) PASS: gdb.base/structs.exp: zed L<n> for finish; return 1 structs-tc
finish
Run till exit from #0  fun1 () at /ironwood1/sourceware-git/msp430-elf/../binutils-gdb/gdb/testsuite/gdb.base/structs.c:125
ERROR: Process no longer exists

gdb/ChangeLog:
    	* infcmd.c (finish_command_fsm_should_stop): Don't attempt to
    	record a NULL value.