Improve error message for segv in read_barrier_handler

If the page_body is a null pointer, then read_barrier_handler will
crash with an unrelated message. This commit improves the error message.

Before:

test.rb:1: [BUG] Couldn't unprotect page 0x0000000000000000, errno: Cannot allocate memory

After:

test.rb:1: [BUG] read_barrier_handler: segmentation fault at 0x14

gc.c

@@ -5210,18 +5210,27 @@ static void invalidate_moved_page(rb_objspace_t *objspace, struct heap_page *pag
 
 #if GC_CAN_COMPILE_COMPACTION
 static void
-read_barrier_handler(uintptr_t address)
+read_barrier_handler(uintptr_t original_address)
 {
     VALUE obj;
     rb_objspace_t * objspace = &rb_objspace;
 
-    address -= address % BASE_SLOT_SIZE;
+    /* Calculate address aligned to slots. */
+    uintptr_t address = original_address - (original_address % BASE_SLOT_SIZE);
 
     obj = (VALUE)address;
 
+    struct heap_page_body *page_body = GET_PAGE_BODY(obj);
+
+    /* If the page_body is NULL, then mprotect cannot handle it and will crash
+     * with "Cannot allocate memory". */
+    if (page_body == NULL) {
+        rb_bug("read_barrier_handler: segmentation fault at 0x%lx", original_address);
+    }
+
     RB_VM_LOCK_ENTER();
     {
-        unlock_page_body(objspace, GET_PAGE_BODY(obj));
+        unlock_page_body(objspace, page_body);
 
         objspace->profile.read_barrier_faults++;