fix: skip SSA analysis on ill-typed packages to prevent panic

[ccojocar]

Summary

Packages with type errors (pkg.IllTyped == true) have partial type information that can cause nil pointer dereferences inside the SSA builder (golang.org/x/tools/go/ssa.emitConv). This results in a panic during gosec scans, even though the panic is caught by defer/recover — users still see a frightening 50+ line stack trace.

Root Cause

When packages.Load processes a package with compilation errors, it still populates pkg.Types and pkg.TypesInfo with partial data. Some TypesInfo.Types entries have nil Type fields for expressions the type checker couldn't resolve. The SSA builder doesn't guard against these nil types in emitConv, causing a nil pointer dereference.

The existing guards in buildSSA check for pkg.Types == nil and pkg.TypesInfo == nil, but miss the case where these fields are populated with incomplete data.

Fix

Add a pkg.IllTyped check in buildSSA (after the existing nil guards) that returns a clean error before attempting SSA construction. AST-based rules continue to run normally on the affected package.

Before:

[gosec] Panic when running SSA analyzer on package: models. Panic: runtime error: invalid memory address or nil pointer dereference
Stack trace:
goroutine 54 [running]:
runtime/debug.Stack()
... (50+ lines)

After:

[gosec] Error building the SSA representation of the package models: package models has type errors, skipping SSA analysis

Changes

• analyzer.go: Add pkg.IllTyped guard in buildSSA
• analyzer_core_internal_test.go: Add TestBuildSSAIllTypedPackage

Fixes #1604

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.02%. Comparing base (1ced32d) to head (2eaae81).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1607   +/-   ##
=======================================
  Coverage   81.01%   81.02%           
=======================================
  Files         108      108           
  Lines       10010    10012    +2     
=======================================
+ Hits         8110     8112    +2     
  Misses       1418     1418           
  Partials      482      482           

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