Bug 1414901 - part 1a - make mozWritePoison respect its documentation; r=Waldo

The documentation for mozWritePoison says that only an even number of
sizeof(uintptr_t) bytes are overwritten; any trailing bytes are not
touched.  This documentation doesn't correspond to how the function
actually works.  The function as written will happily overwrite trailing
bytes and any bytes not contained in the object, if the passed-in size
isn't divisible by sizeof(uintptr_t).  Let's fix that.

mfbt/Poison.h

@@ -39,7 +39,7 @@ inline void mozWritePoison(void* aPtr, size_t aSize)
 {
   const uintptr_t POISON = mozPoisonValue();
   char* p = (char*)aPtr;
-  char* limit = p + aSize;
+  char* limit = p + (aSize & ~(sizeof(uintptr_t) - 1));
   MOZ_ASSERT((uintptr_t)aPtr % sizeof(uintptr_t) == 0, "bad alignment");
   MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
   for (; p < limit; p += sizeof(uintptr_t)) {