Add ASSERT to check m_fd for values <0

We are seeing RNG falures on HURD, but we are not throwing when constructing BlockingRng or NonblockingRng. This is despite the fact that /dev/urandom is missing during testing. NonblockingRng should always thwo when /dev/urandom is missing.

osrng.cpp

@@ -134,6 +134,9 @@ NonblockingRng::NonblockingRng()
 	m_fd = open("/dev/urandom",O_RDONLY);
 	if (m_fd == -1)
 		throw OS_RNG_Err("open /dev/urandom");
+
+	// Do some OSes return -NNN instead of -1?
+	CRYPTOPP_ASSERT(m_fd >= 0);
 #endif
 }
 
@@ -203,6 +206,9 @@ BlockingRng::BlockingRng()
 	m_fd = open(CRYPTOPP_BLOCKING_RNG_FILENAME,O_RDONLY);
 	if (m_fd == -1)
 		throw OS_RNG_Err("open " CRYPTOPP_BLOCKING_RNG_FILENAME);
+
+	// Do some OSes return -NNN instead of -1?
+	CRYPTOPP_ASSERT(m_fd >= 0);
 }
 
 BlockingRng::~BlockingRng()