Increase the internal log buffer.

With the switch to 4K data packets in the Icon HD backend, the internal
log buffer is no longer large enough for hexdumps of these data packets.
A hexdump needs twice the size of the binary data, plus some extra bytes
for the header and the terminating null byte.

src/context.c

@@ -38,7 +38,7 @@ struct dc_context_t {
 	dc_logfunc_t logfunc;
 	void *userdata;
 #ifdef ENABLE_LOGGING
-	char msg[4096];
+	char msg[8192 + 32];
 #ifdef _WIN32
 	DWORD timestamp;
 #else