gdb: Fix backtrace with disassemble-next-line on

gdb: Fix backtrace with disassemble-next-line on

In this commit:

  commit ec8e2b6d3051f0b4b2a8eee9917898e95046c62f
  Date:   Fri Jun 14 23:43:00 2019 +0100

      gdb: Don't allow annotations to influence what else GDB prints

A change was accidentally made that moved a call to do_gdb_disassembly
out of an if block guarded by 'if (source_print && sal.symtab)'.  The
result was that if a user has 'set disassemble-next-line on' then the
backtrace would now include some disassembly of a few instructions in
each frame.

This change was not intentional, but was not spotted by any tests.

This commit restores the old behaviour and adds a test to ensure this
doesn't break again in the future.

gdb/ChangeLog:

	* stack.c (print_frame_info): Move disassemble_next_line code
	inside source_print block.

gdb/testsuite/ChangeLog:

	* gdb.base/backtrace.c: New file.
	* gdb.base/backtrace.exp: New file.

Change-Id: I47c52a202fa74be138382646b695827940178689