Fix setAmxString cutting off the last character of strings

[metita]

Fixes #383.

3c08fde changed setAmxString to treat max as the full buffer size and reserve a cell for the null terminator. But plugins pass charsmax(), which already excludes the null, so every native that returns a string (get_entvar, get_member, ...) ended up dropping the last character.

This reverts the length handling back to the original charsmax behavior.

[metita]

tested on a live 5.29 server, same version so the only diff is the patch.

vanilla:

[A] tight='info_targe' -> FAIL (truncated)
[B] sizeof='info_target' -> correct

patched:

[A] tight='info_target' -> PASS
[B] sizeof='info_target' -> correct

[A] is the check: tight buffer + charsmax, vanilla drops the last char, patched keeps it. sizeof works in both.

test plugin:

#include <amxmodx>
#include <reapi>

public plugin_init()
{
    register_plugin("setAmxString test", "1.0", "test")
    register_srvcmd("test_setamxstr", "cmd_test")
}

public cmd_test()
{
    new ent = rg_create_entity("info_target")
    if (!is_entity(ent)) return PLUGIN_HANDLED;

    new tight[12] // "info_target" = 11 chars, charsmax = 11
    get_entvar(ent, var_classname, tight, charsmax(tight))
    server_print("[A] tight='%s' -> %s", tight, equal(tight, "info_target") ? "PASS" : "FAIL")

    new tight2[12]
    get_entvar(ent, var_classname, tight2, sizeof(tight2))
    server_print("[B] sizeof='%s' -> %s", tight2, equal(tight2, "info_target") ? "correct" : "differs")

    rg_remove_entity(ent)
    return PLUGIN_HANDLED;
}