number of lines of matrices

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear PARI Developers,

Maybe we should add a function to query the number of lines of a non-empty matrix.

There are lot of instances of lg(x[1]) in the code, and this should be replaced
by lg(gel(x,1)) because it causes problem with strict aliasing.

something like
INLINE long
nblin(GEN x) { return lg(gel(x,1))-1; }

But how should we call it ? widthmat() ?

Cheers,
Bill.