Fix a compiler warning with gcc-4.9

gcc-4.9 seems to be able to see that accessing p[1] in 1D is out of bound
for spacedim == 1 but it can omit the warning if accessed inside a switch
statement. Fix this warning (even though we won't use SphericalManifold
for spacedim == 1).
Merge pull request #221 from kronbichler/master

Fix a compiler warning with gcc-4.9. 
gcc-4.9 seems to be able to see that accessing R[1] in 1D is out of bound
for spacedim == 1 but it can omit the warning if accessed inside a switch
statement. Fix this warning (even though we won't use SphericalManifold
for spacedim == 1).