Handle bad ev/thread construction better.

We were incorrectly handling bad arguments to create a new thread
resulting in corruption.

test/fuzzers/fuzz_dobytes.c

@@ -2,7 +2,9 @@
 #include <janet.h>
 
 /* Disable leak sanitizer */
-int __lsan_is_turned_off(void) { return 1; }
+int __lsan_is_turned_off(void) {
+    return 1;
+}
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 

test/suite-ev.janet

@@ -605,4 +605,8 @@
 (assert (zero? exit-code) "subprocess ran")
 (assert (= data "hi\nthere\n") "output is correct")
 
+# Error handling
+(assert-error "bad thread" (ev/thread in))
+(assert-error "bad thread 2" (ev/thread (fn [x y] x) 1))
+
 (end-suite)