Fix running to breakpoint set in inline function by lineno/address

Fix running to breakpoint set in inline function by lineno/address

Commit 61b04dd04ac2 ("Change inline frame breakpoint skipping logic
(fix gdb.gdb/selftest.exp)") caused a GDB crash when you set a
breakpoint by line number in an inline function, and then run to the
breakpoint:

    $ gdb -q test Reading symbols from test...done.
    (gdb) b inline-break.c:32
    Breakpoint 1 at 0x40062f: file inline-break.c, line 32.
    (gdb) run
    Starting program: /[...]/test
    [1]    75618 segmentation fault  /[...]/gdb -q test

The problem occurs because we assume that a bp_location's symbol is
not NULL, which is not true when we set the breakpoint with a linespec
location:

    Program received signal SIGSEGV, Segmentation fault.
    0x00000000006f42bb in stopped_by_user_bp_inline_frame (
        stop_chain=<optimized out>, frame_block=<optimized out>)
        at gdb/inline-frame.c:305
    305		      && frame_block == SYMBOL_BLOCK_VALUE (loc->symbol))
    (gdb) p loc->symbol
    $1 = (const symbol *) 0x0

The same thing happens if you run to a breakpoint set in an inline
function by address:

  (gdb) b *0x40062f
  Breakpoint 3 at 0x40062f: file inline-break.c, line 32.

To fix this, add a null pointer check, to avoid the crash, and make it
so that if there's no symbol for the location, then we present the
stop at the inline function.  This preserves the previous behavior
when e.g., setting a breakpoint by address, with "b *ADDRESS".

gdb/ChangeLog:
2018-06-29  Pedro Alves  <palves@redhat.com>

	* inline-frame.c (stopped_by_user_bp_inline_frame): Return
	true if the the location has no symbol.

gdb/testsuite/ChangeLog:
2018-06-29  Pedro Alves  <palves@redhat.com>

	* gdb.opt/inline-break.c (func1): Add "break here" marker.
	* gdb.opt/inline-break.exp: Test setting breakpoints by line
	number and address and running to them.