Add regression test for cbmc crash in flattening

Author: danpoe

regression/cbmc/r_w_ok_bug/main.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+#include <stdlib.h>
+
+void main()
+{
+  char *p1;
+  __CPROVER_assume(__CPROVER_r_ok(p1 - 1, 1));
+  *p1;
+}

regression/cbmc/r_w_ok_bug/test.desc

@@ -0,0 +1,9 @@
+KNOWNBUG
+main.c
+--pointer-check --no-simplify --no-propagation
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+Crashes during the flattening