[gdb/tui] Fix len_without_escapes in tui-disasm.c

[gdb/tui] Fix len_without_escapes in tui-disasm.c

On openSUSE Tumbleweed I run into:
...
FAIL: gdb.tui/basic.exp: asm window shows main
ERROR: invalid command name "_csi_L"
...

Using a minimal example, we get:
...
$ gdb -q outputs/gdb.tui/basic/basic -ex "tui enable" -ex "layout asm"
<TUI output>
src/gdb/ui-style.c:243: internal-error: bool \
  ui_file_style::parse(const char*, size_t*): Assertion `match == 0' failed.
...

The problem is in len_without_escapes, where we detect the start of an escape
sequence, but then pass ptr to style.parse while ptr no longer points to the
escape due to the ptr++ in the while condition:
...
  while ((c = *ptr++) != '\0')
     {
      if (c == '\033')
        {
          ui_file_style style;
          size_t n_read;
          if (style.parse (ptr, &n_read))
...

Fix this by removing the ++ in the while condition, and adding ptr++ in the
loop body where appropriate.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-04-06  Tom de Vries  <tdevries@suse.de>

	PR tui/27680
	* tui/tui-disasm.c (len_without_escapes): Pass ptr pointing at escape
	to style.parse.