fix(math): avoid nil internal big.Int when Unmarshal receives empty i…

[neyy91]

Description

Bug

LegacyDec, Int and Uint Unmarshal (math/legacy_dec.go:868,
int.go:493, uint.go:198) return a nil error on
empty input while leaving the receiver's internal big.Int nil — the d = nil
statement only reassigns the function-local pointer copy, not the caller's
value. This is asymmetric with UnmarshalJSON, which sets d.i = new(big.Int).
IsZero/IsNegative then dereference the nil internal value and panic.

Reachability (untrusted protobuf)

A DecCoin/Coin with a zero-length Amount field is valid wire data: the
generated decoder (coin.pb.go:596) calls
m.Amount.Unmarshal(dAtA[iNdEx:iNdEx]) — an empty slice — so decode succeeds
with a nil-internal Amount. DecCoin.Validate() (dec_coin.go:140)
then calls IsNegative() with no IsNil guard → nil pointer panic.
Honest encoders never emit this (MarshalTo always writes ≥1 byte), so only
crafted/malformed input is affected.

Fix

Initialize the internal big.Int to zero on empty input, matching
UnmarshalJSON. Empty input now decodes to a valid zero instead of a
panic-prone nil.

Test

TestUnmarshalEmptyIsZeroNotNil (all three types): before — IsZero/IsNegative
panic on the nil internal value; after — empty input is a usable zero.
TestUnmarshalZeroRoundTrip (all three types): a zero value survives a
Marshal/Unmarshal round trip as a usable zero. Full math module suite
green, go vet clean.

[greptile-apps]

PR author is not in the allowed authors list.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.06%. Comparing base (c591a9f) to head (0ef4108).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26536      +/-   ##
==========================================
+ Coverage   64.83%   65.06%   +0.22%     
==========================================
  Files         784      789       +5     
  Lines       55096    56035     +939     
==========================================
+ Hits        35724    36461     +737     
- Misses      19372    19574     +202     

Files with missing lines	Coverage Δ
math/int.go	88.80% <100.00%> (ø)
math/legacy_dec.go	84.02% <100.00%> (ø)
math/uint.go	93.07% <100.00%> (ø)

... and 7 files with indirect coverage changes

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