Ensure -maxsigcachesize is in valid range

- If the -maxsigcachesize parameter is set to zero, setup a minimum sized
sigcache (2 elements) rather than segfaulting.
- Handle maxsigcachesize being negative
- Handle maxsigcachesize being too large

src/script/sigcache.cpp

@@ -93,8 +93,9 @@ static CSignatureCache signatureCache;
 // To be called once in AppInit2/TestingSetup to initialize the signatureCache
 void InitSignatureCache()
 {
-    size_t nMaxCacheSize = GetArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
-    if (nMaxCacheSize <= 0) return;
+    // nMaxCacheSize is unsigned. If -maxsigcachesize is set to zero,
+    // setup_bytes creates the minimum possible cache (2 elements).
+    size_t nMaxCacheSize = std::min(std::max((int64_t)0, GetArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE)), MAX_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
     size_t nElems = signatureCache.setup_bytes(nMaxCacheSize);
     LogPrintf("Using %zu MiB out of %zu requested for signature cache, able to store %zu elements\n",
             (nElems*sizeof(uint256)) >>20, nMaxCacheSize>>20, nElems);

src/script/sigcache.h

@@ -14,6 +14,8 @@
 // systems). Due to how we count cache size, actual memory usage is slightly
 // more (~32.25 MB)
 static const unsigned int DEFAULT_MAX_SIG_CACHE_SIZE = 32;
+// Maximum sig cache size allowed
+static const int64_t MAX_MAX_SIG_CACHE_SIZE = 16384;
 
 class CPubKey;