sys-file-reader: Fix memory leak.

sys-file-reader: Fix memory leak.

The C standards say that the compiler is allowed to optimize away
changes to local variables within a function between a call to
setjmp() and a later call to longjmp(), unless the local variables
are volatile-qualified.

The 'dict' local variable in sfm_open_reader() fits this
description but wasn't volatile-qualified.  GCC in fact optimized
out the changes on my system, and this commit fixes that.