trace: Initialize USDT arguments to 0 before reading (iovisor#725)

Fixes iovisor#722, in which a USDT probe that has more than
one location and the type of the argument is a string
caused trace to potentially access an uninitialized
stack variable, thereby not passing BPF program
verification at load time.

tools/trace.py

@@ -301,7 +301,7 @@ def _generate_field_assign(self, idx):
  expr = self.values[idx].strip()
  text = ""
  if self.probe_type == "u" and expr[0:3] == "arg":
- text = (" u64 %s;\n" +
+ text = (" u64 %s = 0;\n" +
  " bpf_usdt_readarg(%s, ctx, &%s);\n") % \
  (expr, expr[3], expr)