guc: make dereference style consistent in check_backtrace_functions

Hi,

In check_backtrace_functions(), most accesses to the input string follow
the pattern (*newval)[i]. However, the empty-string check is currently
written as:

if (*newval[0] == '\0')

While functionally correct due to how the compiler handles the
address-of-address context here, this form is semantically misleading. It
relies on implicit operator precedence rather than explicit intent.

The attached patch rewrites it as:

if ((*newval)[0] == '\0')

This change ensures semantic clarity and maintains a consistent
dereferencing style throughout the function. No functional changes are
introduced.

Regards,
Zhang Hu