Fix bug in OpenMP parallelization.

m is declared outside the parallelized for-loops which makes it shared
by default according the OpenMP standard as well as by the
"default(shared)" statement.  This implies a race condition and the
actual value of m is undefined with in the loops.  The patch fixes this
issue.

Note that the compiler might not allocate storage for m on the stack and
use register instead, so the race condition may not manifest in wrong
output when optimization is enabled, but the error is present anyway.