Fixed maybe-uninitialized warning in error creation.

[xeioex]

Ensuring that buf is always initialized in njs_throw_error_va() and njs_error_fmt_new(), by requiring fmt to always be non NULL.

This fixes GCC warnings like:
169 | njs_unicode_decode_t ctx;
| ^
In function ‘njs_utf8_length’,
inlined from ‘njs_error_new’ at src/njs_error.c:39:14,
inlined from ‘njs_throw_error_va’ at src/njs_error.c:69:5:
src/njs_utf8.h:141:12: error: ‘buf’ may be used uninitialized
[-Werror=maybe-uninitialized]
141 | return njs_utf8_stream_length(&ctx, p, len, 1, 1, NULL);

Proposed changes

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes

[VadimZhestikov]

Looks good, approved, The only small thought is about error messages:

Below collected Error messages used by different engines. In this cases chrome provides bit more info to simplify debugging.
May be it would be nice behave like chrome?

String.fromCodePoint('_')
njs: RangeError: invalid code point
qjs: RangeError: invalid code point
node: RangeError: Invalid code point NaN
chrome: RangeError: Invalid code point NaN
firefox: RangeError: NaN is not a valid code point

'abc'.repeat(Infinity)
njs: RangeError: invalid count value
qjs: RangeError: invalid repeat count
node: RangeError: Invalid count value
chrome: RangeError: Invalid count value: Infinity
firefox: RangeError: repeat count must be less than infinity and not overflow maximum string size

''.repeat(-1)
njs: RangeError: invalid count value
qjs: RangeError: invalid repeat count
node: RangeError: Invalid count value
chrome: RangeError: Invalid count value: -1
FireFox: RangeError: repeat count must be non-negative

[xeioex]

@VadimZhestikov

node: RangeError: Invalid code point NaN

I thought about it, but it requires calling of toString() for a invalid value just for an exception, not worth the effort I think.