Don't allocate less memory than required

When switching from a simple to a sparse array,
keep the previously allocated size, to not corrupt
memory.

Change-Id: I33f0fb049a2ad6f24ee3703f2c333855830fe9d2
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

src/qml/jsruntime/qv4arraydata.cpp

@@ -104,6 +104,8 @@ void ArrayData::realloc(Object *o, Type newType, uint requested, bool enforceAtt
 
         if (requested <= d->alloc() && newType == d->type() && hasAttrs == enforceAttributes)
             return;
+        if (alloc < d->alloc())
+            alloc = d->alloc();
 
         if (d->type() < Sparse) {
             offset = static_cast<SimpleArrayData *>(d)->d()->offset;