Fix --nondet-volatile bug where lhs was not considered in assignment

Author: danpoe

regression/goto-instrument/nondet-volatile-01/test.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+
+void main()
+{
+  int a[2] = {0};
+
+  volatile int i = 0;
+  a[i] = 1;
+
+  assert(a[1] == 0); // should fail
+}

regression/goto-instrument/nondet-volatile-01/test.desc

@@ -0,0 +1,9 @@
+CORE
+test.c
+--nondet-volatile
+^EXIT=10$
+^SIGNAL=0$
+--
+--
+Check that volatile variables appearing in the lhs of an assignment are
+correctly treated as nondet

src/goto-instrument/nondet_volatile.cpp

@@ -88,7 +88,7 @@ void nondet_volatile(
     if(instruction.is_assign())
     {
       nondet_volatile_rhs(symbol_table, to_code_assign(instruction.code).rhs());
-      nondet_volatile_lhs(symbol_table, to_code_assign(instruction.code).rhs());
+      nondet_volatile_lhs(symbol_table, to_code_assign(instruction.code).lhs());
     }
     else if(instruction.is_function_call())
     {