fix: null-deref when dbgKey unset under HELIB_DEBUG (#501)

[BAder82t]

Summary

Fixes segfault reported in #501.

When HElib is compiled with `HELIB_DEBUG` and the user program does not call `setupDebugGlobals(&sk, ...)`, the global `dbgKey` / `dbgEa` remain `nullptr`. Several debug-only call sites dereference `*dbgKey` / `*dbgEa` unconditionally, causing a segfault. The reporter hit this running the bundled `BGV_binary_arithmetic` example:

```
SEGV at keys.cpp:273 PubKey::getContext(this=0x0)
<- SecKey::Decrypt(this=0x0, ...) keys.cpp:1338
<- embeddingLargestCoeff debugging.cpp:60
<- realToEstimatedNoise debugging.cpp:30
<- checkNoise debugging.cpp:50
<- Ctxt::multiplyBy Ctxt.cpp:1772
```

Fix

Guard every `*dbgKey` / `*dbgEa` dereference with an explicit null check, matching the existing pattern already used in `extractDigits.cpp` and `debugging.cpp`. Release builds (`HELIB_DEBUG` off) are unchanged.

Call sites touched:

• `src/Ctxt.cpp` — `Ctxt::multiplyBy` (the reported site)
• `src/binaryCompare.cpp` — both debug blocks in `compProducts` / `compEqGt`
• `src/binaryArith.cpp` — `multByNegative`, `multTwoNumbers`, both blocks in `AddDAG::printAddDAG`, and `decryptAndSum`

Verification

Minimal BGV repro (single `Ctxt::multiplyBy`, no `setupDebugGlobals` call, built with `-DHELIB_DEBUG -fsanitize=address`):

Build	Result
unpatched master	`SEGV keys.cpp:273` (exit 134) — exact stack from #501
this PR	`multiplyBy completed` — exit 0

Closes

#501