Gdb returns stack level of -1 in some cases

I suggest we open a bug and do the fix. Treat the -1 as a zero I suppose; maybe just ignore the thread? Will have to try with the sample project to see if we can repro the behavior though.

(8240) ->1009^done,threads=[{id="10",target-id="Thread 936",frame={level="0",addr="0x40036fc0",func="__ioctl",args=[],from="libc.so"},state="stopped",core="0"},{id="9",target-id="Thread 935",frame={level="0",addr="0x40036fc0",func="__ioctl",args=[],from="libc.so"},state="stopped",core="0"},{id="8",target-id="Thread 931",frame={level="0",addr="0x400380e0",func="__futex_syscall3",args=[],from="libc.so"},state="stopped",core="0"},{id="7",target-id="Thread 930",frame={level="0",addr="0x400380e0",func="__futex_syscall3",args=[],from="libc.so"},state="stopped",core="0"},{id="6",target-id="Thread 929",frame={level="0",addr="0x400380e0",func="__futex_syscall3",args=[],from="libc.so"},state="stopped",core="0"},{id="5",target-id="Thread 928",frame={level="0",addr="0x400380e0",func="__futex_syscall3",args=[],from="libc.so"},state="stopped",core="0"},{id="4",target-id="Thread 927",frame={level="0",addr="0x40037ba8",func="recvmsg",args=[],from="libc.so"},state="stopped",core="0"},{id="3",target-id="Thread 926",frame={level="0",addr="0x400379a0",func="__rt_sigtimedwait",args=[],from="libc.so"},state="stopped",core="0"},{id="2",target-id="Thread 924",frame={level="0",addr="0x400380e0",func="__futex_syscall3",args=[],from="libc.so"},state="stopped",core="0"},{id="1",target-id="Thread 921",frame={level="-1",addr="0x40757610",func="??",args=[]},state="stopped",core="0"}],current-thread-id="1"

Start with the sample project mentioned in the issue and reproduce the GDB MI thread response containing frame level "-1". Determine the expected handling for that stack level and verify the behavior against the captured MI output; no source file or test is identified in the issue.