fix issue 829

Needed to make array initialized to null before bpf_probe_read as the
verifier was rejecting the original code. will probably need to fix the
tutorial as well

examples/tracing/nodejs_http_server.py

@@ -23,7 +23,7 @@
 #include <uapi/linux/ptrace.h>
 int do_trace(struct pt_regs *ctx) {
  uint64_t addr;
- char path[128];
+ char path[128]={0};
  bpf_usdt_readarg(6, ctx, &addr);
  bpf_probe_read(&path, sizeof(path), (void *)addr);
  bpf_trace_printk("path:%s\\n", path);