Allow the "match" function to accept pid=0 as an argument.

Because the function was initializing pid as 0, when the user passed pid=0 as an argument it was indistinguishable from not pid being passed at all.

Closes VirusTotal/yara#1480

yara-python.c

@@ -1499,7 +1499,7 @@ static PyObject* Rules_match(
  char* filepath = NULL;
  Py_buffer data = {0};
 
- int pid = 0;
+ int pid = -1;
  int timeout = 0;
  int error = ERROR_SUCCESS;
  int fast_mode = 0;
@@ -1535,7 +1535,7 @@ static PyObject* Rules_match(
  &callback_data.which,
  &callback_data.warnings_callback))
  {
- if (filepath == NULL && data.buf == NULL && pid == 0)
+ if (filepath == NULL && data.buf == NULL && pid == -1)
  {
  return PyErr_Format(
  PyExc_TypeError,
@@ -1646,7 +1646,7 @@ static PyObject* Rules_match(
 
  Py_END_ALLOW_THREADS
  }
- else if (pid != 0)
+ else if (pid != -1)
  {
  callback_data.matches = PyList_New(0);
 
@@ -1686,7 +1686,7 @@ static PyObject* Rules_match(
  {
  handle_error(error, filepath);
  }
- else if (pid != 0)
+ else if (pid != -1)
  {
  handle_error(error, "<proc>");
  }