PR 7071: Optional $context variables fix

* tapsets.cxx (dwarf_var_expanding_visitor::visit_target_symbol):
  Substitute erroneous target symbol with literal 0 if session level
  flag, skip_badvars is set.
* session.h (struct systemtap_session):
  New flag: skip_badvars.
* main.cxx:
  Command line argument --skip-badvars added.
* stap.1.in:
  Entry for new option --skip-badvars.
* NEWS:
  Added blurb for new option now available.
* testsuite/semok/badvar.stp:
  Test case to check added functionality.

1 files changed, 7 insertions, 0 deletions

diff --git a/testsuite/semok/badvar.stp b/testsuite/semok/badvar.stp
new file mode 100755
index 00000000..b3bd2d67
--- /dev/null
+++ b/testsuite/semok/badvar.stp
@@ -0,0 +1,7 @@
+#! stap --skip-badvars
+
+probe syscall.read {
+  if ($foo == 0)
+    printf ("Voila! It works..\n")
+  exit ()
+}