update mser.c  fix the height of a tree

vl_mser_pix nr_val = 0 ;
vl_uint     nr_idx = 0 ;
int         hgt   = r [ r_idx] .height ;
int         n_hgt = r [nr_idx] .height ;
r_idx = climb(r,   idx) ;
nr_idx = climb(r, n_idx) ;

when i debug the mser.c. i found the "n_hgt","hgt"of the subtree is not always correct. then i found the code below. it means that the "n_hgt" is always the height of r[0]!!.so i modify it.