prng: treat unsuccessful init state

src/common/prng.c

@@ -97,6 +97,9 @@ double tls_rand()
 #endif
 		dsfmt_init_gen_rand(s, seed);
 		(void)pthread_setspecific(tls_prng_key, s);
+	} else {
+		fprintf(stderr, "error: PRNG: can't set thread state\n");
+		return .0;
 	}
 
 	return dsfmt_genrand_close_open(s);