Fix a bug in DetectStructure

The logic for determing static/dynamic f-block size in
DetectStructure was broken in a corner case, where the very first
row block which was used to initialize the f_block_size contained
more than one f blocks of varying sizes. The way the if block
was structured, no iteration was performed on the remaining
f-blocks and the loop failed to detect that the f-block size
was actually changing.

If in the remaining row blocks, there were no row blocks
with varying f-block sizes, the function will erroneously
return a static f-block size.

Thanks to Johannes Schonberger for providing a reproduction for this
rather tricky corner case.

Change-Id: Ib442a041d8b7efd29f9653be6a11a69d0eccd1ec