test: add windows-only snprintf() function

Should fix the build after 96f32a2 inadvertently broke it.

There is no snprintf() on Windows because, hey, it's a C99 addition
and the people from Redmond, WA are still firmly stuck in 1989.

test/task.h

@@ -153,4 +153,32 @@ enum test_status {
     return TEST_SKIP;                                                         \
   } while (0)
 
+#ifdef _WIN32
+
+#include <stdarg.h>
+
+/* Emulate snprintf() on Windows, _snprintf() doesn't zero-terminate the buffer
+ * on overflow...
+ */
+static int snprintf(char* buf, size_t len, const char* fmt, ...) {
+  va_list ap;
+  int n;
+
+  va_start(ap, fmt);
+  n = _vsprintf_p(buf, len, fmt, ap);
+  va_end(ap);
+
+  /* It's a sad fact of life that no one ever checks the return value of
+   * snprintf(). Zero-terminating the buffer hopefully reduces the risk
+   * of gaping security holes.
+   */
+  if (n < 0)
+    if (len > 0)
+      buf[0] = '\0';
+
+  return n;
+}
+
+#endif
+
 #endif /* TASK_H_ */