[amg][bugfix] Fix dirichlet processing in FastAMG

The comment and surroundig code indicate that FastAMG should
solve during pre() for dirichlet entries. This is determined
by checking if a row does contain a diagonal entry while
all other entries are zero.

The check for this was buggy because hasDiagonal was always false.
Now it's set to true if a diagonal entry is found and nonzero.