[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.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2021-02-05  Tom de Vries  <tdevries@suse.de>
+
+	PR breakpoints/27313
+	* break-catch-syscall.c (catch_syscall_split_args): Reject negative
+	syscall numbers.
+
 2021-02-05  Tom Tromey  <tom@tromey.com>
 
 	* compile/compile-c-support.c (get_compile_context)

gdb/break-catch-syscall.c

@@ -390,6 +390,8 @@ catch_syscall_split_args (const char *arg)
       syscall_number = (int) strtol (cur_name, &endptr, 0);
       if (*endptr == '\0')
 	{
+	  if (syscall_number < 0)
+	    error (_("Unknown syscall number '%d'."), syscall_number);
 	  get_syscall_by_number (gdbarch, syscall_number, &s);
 	  result.push_back (s.number);
 	}

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2021-02-05  Tom de Vries  <tdevries@suse.de>
+
+	PR breakpoints/27313
+	* gdb.base/catch-syscall.exp: Check that "catch syscall -1" is
+	rejected.
+
 2021-02-05  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.dwarf2/main-subprogram.exp: Add KFAIL for PR symtab/24549.

gdb/testsuite/gdb.base/catch-syscall.exp

@@ -54,6 +54,9 @@ gdb_test_multiple "continue" $test {
     }
 }
 
+# Test-case for PR27313.  Verify that negative syscall numbers are refused.
+gdb_test "catch syscall -1" "Unknown syscall number '-1'\\."
+
 # All (but the last) syscalls from the example code.  It is filled in
 # proc setup_all_syscalls.
 set all_syscalls { }