[gdb/breakpoints] Fix segfault for catch syscall -1

[gdb/breakpoints] Fix segfault for catch syscall -1

Using a hello world a.out, I run into a segfault:
...
$ gcc hello.c
$ gdb -batch a.out -ex "catch syscall -1" -ex r
Catchpoint 1 (syscall -1)
Aborted (core dumped)
...

Fix this by erroring out if a negative syscall number is used in the
catch syscall command.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27313
	* break-catch-syscall.c (catch_syscall_split_args): Reject negative
	syscall numbers.

gdb/testsuite/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27313
	* gdb.base/catch-syscall.exp: Check that "catch syscall -1" is
	rejected.