Revert addition of padding at the start of BagHeader

There was a bug in this code -- when we shrink a bag with
ResizeBag, the new space may only be a single word, so the type
and flags members of BagHeader must be in the first word.

src/gasman.c

@@ -472,9 +472,7 @@ static void CANARY_ALLOW_ACCESS_BAG(Bag bag)
 
     BagHeader * header = BAG_HEADER(bag);
     VALGRIND_MAKE_MEM_DEFINED(
-        (char *)header + sizeof(header->memory_canary_padding1),
-        sizeof(*header) - sizeof(header->memory_canary_padding1) -
-            sizeof(header->memory_canary_padding2));
+        header, sizeof(*header) - sizeof(header->memory_canary_padding));
 }
 
 // Reverse CANARY_ALL_ACCESS_BAG, making the masterpointer, bag contents and
@@ -488,9 +486,7 @@ static void CANARY_FORBID_ACCESS_BAG(Bag bag)
 
     BagHeader * header = BAG_HEADER(bag);
     VALGRIND_MAKE_MEM_NOACCESS(
-        (char *)header + sizeof(header->memory_canary_padding1),
-        sizeof(*header) - sizeof(header->memory_canary_padding1) -
-            sizeof(header->memory_canary_padding2));
+        header, sizeof(*header) - sizeof(header->memory_canary_padding));
 }
 
 // Mark all bags as accessible

src/gasman.h

@@ -77,11 +77,6 @@ typedef UInt * *        Bag;
 **
 */
 typedef struct {
-#if defined(GAP_MEMORY_CANARY)
-    // The following variable is marked as not readable or writable
-    // in valgrind, to check for code reading before the start of the header.
-    uint64_t memory_canary_padding1[8];
-#endif
     uint8_t type : 8;
     uint8_t flags : 8;
     // the following unnamed field ensures that on 32 bit systems,
@@ -94,7 +89,7 @@ typedef struct {
 #if defined(GAP_MEMORY_CANARY)
     // The following variable is marked as not readable or writable
     // in valgrind, to check for code reading before the start of a Bag.
-    uint64_t memory_canary_padding2[8];
+    uint64_t memory_canary_padding[8];
 #endif
 } BagHeader;