[gdb/tui] Make assert in tui_find_disassembly_address more strict

[gdb/tui] Make assert in tui_find_disassembly_address more strict

In tui_find_disassembly_address we find an assert:
...
      if (asm_lines.size () < max_lines)
	{
	  if (!possible_new_low.has_value ())
	    return new_low;

	  /* Take the best possible match we have.  */
	  new_low = *possible_new_low;
	  next_addr = tui_disassemble (gdbarch, asm_lines, new_low, max_lines);
	  last_addr = asm_lines.back ().addr;
	  gdb_assert (asm_lines.size () >= max_lines);
	}
...

The comment right above:
...
      /* If we failed to disassemble the required number of lines then the
	 following walk forward is not going to work, it assumes that
	 ASM_LINES contains exactly MAX_LINES entries.  Instead we should
	 consider falling back to a previous possible start address in
	 POSSIBLE_NEW_LOW.  */
...
claims that the more strict asm_lines.size () == max_line is required.

Update the assert to reflect this, and move it to after the if because it's
supposed to hold in general, not just when entering the if.

Tested on x86_64-linux.