[GC] fix GC segfault with --optimize

A wrong PARROT_CANNOT_RETURN_NULL in Parrot_pf_get_current_code_segment
(unchecked return of inter->code, which can be null) causes GC segfaults
when inter->code == NULL.

Fixes GH #1186.
This wrong PARROT_CANNOT_RETURN_NULL was added with 3.6.0
in 7dc0e22 at Wed Jun 29 20:52:24 2011
but started being fatal after the new packfile refactor with 7.0,
when the cc optimizer started optimizing away the bc != NULL check
in mark_code_segment()

include/parrot/packfile.h

@@ -824,7 +824,7 @@ PMC * Parrot_pf_annotations_lookup(PARROT_INTERP,
         __attribute__nonnull__(2);
 
 PARROT_PURE_FUNCTION
-PARROT_CANNOT_RETURN_NULL
+PARROT_CAN_RETURN_NULL
 PackFile_ByteCode * Parrot_pf_get_current_code_segment(PARROT_INTERP)
         __attribute__nonnull__(1);
 

src/gc/mark_sweep.c

@@ -282,9 +282,9 @@ mark_code_segment(PARROT_INTERP)
 {
     ASSERT_ARGS(mark_code_segment)
     int i;
-    PackFile_ByteCode   *bc = Parrot_pf_get_current_code_segment(interp);
+    PackFile_ByteCode *bc = Parrot_pf_get_current_code_segment(interp);
 
-    if (bc != NULL) {
+    if (bc) {
         PackFile_ConstTable *ct = bc->const_table;
 
         for (i = 0; i < ct->pmc.const_count; i++) {

src/packfile/api.c

@@ -1536,7 +1536,7 @@ Get's the interpreter's currently active bytecode segment
 */
 
 PARROT_PURE_FUNCTION
-PARROT_CANNOT_RETURN_NULL
+PARROT_CAN_RETURN_NULL
 PackFile_ByteCode *
 Parrot_pf_get_current_code_segment(PARROT_INTERP)
 {