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.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;