small fix to malloc.c, tests 1-5 now pass

test6 should be updated so that the constructor function
does something unsafe, since we now enable SECCOMP mode
before executing constructor functions.

The fact that test7 is failing seems to indicate that scanf
is causing a disallowed system call (probably fstat).
We might need to make a call to scanf before entering
SECCOMP.  Probably could set stdin to be nonblocking
temporarily.