Fixes issue with wrapped script bases.

[zorkow]

This is a fix for the issue that rendering of script elements with an base element that is wrapped in a TexAtom fails.
E.g., ${\bf a}^b$ fails, while $a^b$ works. But the latter only by accident.

The problem is in the getBaseCore method, where we loop into the base element. However, the loop works effectively on the node and not the wrapper, thus we have to reset both core and node, otherwise we end up with the innermost text node and an error in the baseCharZero method.

[dpvc]

OOPS! Good catch. Thanks!