Fix NULL pointer deref when built with gcc -O1 or -O2

The C++ spec says "this" is always NON-NULL, recent versions of gcc will warn
about this and optimizes the "if (this)" we use in some places away:
"warning: nonnull argument ‘this’ compared to NULL [-Wnonnull-compare]"

We rely on "if (this)" checks in several places and refactoring this
is non trivial, so this commit adds a workaround using a helper function
for (this == NULL) checks which is marked as __attribute__((optimize("O0")))
when building with gcc.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

src/lisp/lisp.cpp

@@ -860,7 +860,7 @@ size_t LList::GetLength()
     size_t ret = 0;
 
 #ifdef TYPE_CHECKING
-    if (this && item_type(this) != (ltype)L_CONS_CELL)
+    if (item_type(this) != (ltype)L_CONS_CELL)
     {
         Print();
         lbreak(" is not a sequence\n");
@@ -1268,7 +1268,7 @@ void LObject::Print()
     switch (item_type(this))
     {
     case L_CONS_CELL:
-        if (!this)
+        if (ptr_is_null(this))
         {
             lprint_string("nil");
         }
@@ -3068,7 +3068,7 @@ LObject *LObject::Eval()
 
     LObject *ret = NULL;
 
-    if (this)
+    if (!ptr_is_null(this))
     {
         switch (item_type(this))
         {

src/lisp/lisp.h

@@ -250,7 +250,28 @@ class Lisp
 
 static inline LObject *&CAR(void *x) { return ((LList *)x)->m_car; }
 static inline LObject *&CDR(void *x) { return ((LList *)x)->m_cdr; }
-static inline ltype item_type(void *x) { if (x) return *(ltype *)x; return L_CONS_CELL; }
+
+#ifdef __GNUC__
+/*
+ * C++ spec says "this" is always NON-NULL, recent versions of gcc will warn
+ * about this and optimizes the "if (this)" we use in some places away:
+ * "warning: nonnull argument ‘this’ compared to NULL [-Wnonnull-compare]"
+ * We rely on "if (this)" checks in several places and refactoring this is
+ * non trivial. So we use this little helper marked with
+ * __attribute__((optimize("O0"))) to workaround this.
+ */
+static inline bool __attribute__((optimize("O0"))) ptr_is_null(void *ptr)
+{
+    return ptr == NULL;
+}
+#else
+static inline bool ptr_is_null(void *ptr)
+{
+    return ptr == NULL;
+}
+#endif
+
+static inline ltype item_type(void *x) { if (!ptr_is_null(x)) return *(ltype *)x; return L_CONS_CELL; }
 
 void perm_space();
 void tmp_space();