For a cell of row vectors as in your case, this will pad vectors with zeros to form a matrix
1 2 3 0 0
1 2 3 4 5
1 2 3 4 0
A similar question was asked earlier today, and although the question was worded slightly differently, my answer basically does what you want.
Copying the relevant parts here, a cell of uneven column vectors can be zero padded into a matrix as:
maxLength is assumed to be known. In your case, you have row vectors, which is just a slight modification from this.
maxLength is not known, you can get it as