encoding/prototext: enforce RecursionLimit in skipValue/skipMessageValue

[al4an444]

Summary

skipValue and skipMessageValue in encoding/prototext/decode.go are mutually recursive but do not check RecursionLimit. This allows deeply nested unknown fields to cause a stack overflow when DiscardUnknown is true, completely bypassing the recursion limit that unmarshalMessage and unmarshalMap enforce (added in commit 20262ed0).

Root Cause

• skipValue (line 735) calls skipMessageValue on MessageOpen
• skipMessageValue (line 770) calls skipValue on Name
• Neither function decrements or checks opts.RecursionLimit
• Reached via unmarshalMessage:206 when DiscardUnknown=true or ReservedNames().Has(name)

Reproduction

go test -v with a payload of 5000 nested unknown fields and RecursionLimit=10:

• Skip path: accepted with err=nil (BUG - limit bypassed)
• Known path (Struct/Value): correctly returns exceeded maximum recursion depth

With depth=5000000, the Go runtime aborts with fatal error: stack overflow (not recoverable via recover()).

Fix

Add the same RecursionLimit decrement idiom to both skipValue and skipMessageValue. Since decoder is passed by value, the decrement is naturally scoped per call chain.

This is a 6-line change (3 lines per function) using the exact same pattern as unmarshalMessage and unmarshalMap.

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[al4an444]

@googlebot I signed it!

[puellanivis]

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