The will is not effective and probate has no jurisdiction until the principal dies. Your grandmother can remove anything she wants from the safe deposit box. If your grandmother has given permssion for your mother to enter the safe deposit box and remove the contents before she dies, she can do so. If the savings bond is in your mother's name, your mother can cash the bond.
Attorney Tylman gave you excellent advice. As he mentions, even after your Grandmother passes, if the bonds are MADE OUT TO your mother, then they are hers. If they are made out to your mother AND your grandmother, then they will belong to the survivor of either of them. If they are made out to your mother OR your grandmother then either one can cash them without the others signature. So long as your mother is a signatory on the safe deposit box and that has not been revoked by your grandmother, then she may access the box without penalty. If your mother waits until after your grandmother passes to open the box, then the contents become part of the probate estate and will be potentially tied up until after an inventory has been done.
My answer is of a general nature and should not be construed to be legal advice nor creating an attorney-client relationship. Carol Johnson Law Firm, P.A. practices in the area of Wills, Trusts, and Estates, Disability - with a particular focus on providing Special Needs Trusts for disabled children and adults.