To develop this problem we must apply the concepts related to the Ampere Law. The line integral of B.ds around any closed path is equal to the current permeability constant, this current for the particular case passes through the 'internal' surface delimited by the closed path.
Ampere laws is defined as,
[tex]\oint Bds=mu_0 I[/tex]
The radius of the circle is r, so if
r < R
And there is no current inside so ,
[tex]\oint Bds=mu_0 I[/tex]
[tex]B (2\pi R) = 0[/tex]
Therefore the magnetic field inside the wall is Zero.