Avoid search tree explosion in qsearch

Avoid search tree explosion in qsearch

Under some rare cases we can have a search tree explosion
due to a perpetual check or to a very long non-capture TT
sequence.

This avoids the tree explosion not following TT moves that
are not captures or promotions when we are below the
'generate checks' depth.

Idea suggested by Richard Vida.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>