Fix stack buffer overflow in IPv6 parsing with IPv4 dotted notation

[rustyconover]

Summary

• When parsing IPv6 addresses containing IPv4 dotted decimal notation (e.g., a:b:c:d:e:f:a:1.2.3.4), the IPv4 portion writes two quibbles into a stack-allocated array of size 8. The loop condition only checks parsed_quibble_count < 8, so if we enter with count=7, the first write at index 7 is valid but the second at index 8 overflows the stack buffer.
• This causes a crash (SIGABRT) on malformed input. While 7 hex quibbles followed by IPv4 notation is invalid, the parser should reject it gracefully instead of crashing.
• Fix adds a bounds check (parsed_quibble_count + 2 > IPV6_NUM_QUIBBLE) before writing the two IPv4-derived quibbles, returning an appropriate error message.

Reproduction

LOAD inet;
SELECT 'a:b:c:d:e:f:a:1.2.3.4'::INET;  -- crashes with SIGABRT (exit 134)

Test plan

• Added test case verifying the malformed input is rejected with statement error
• Verify a:b:c:d:e:f:1.2.3.4 (6 quibbles + IPv4 = 8 total) still works correctly
• Verify ::ffff:1.2.3.4 still works correctly

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