[PR] instr: change assert to allow FD 0 return by __open()

Summary: In some cases __open() is returning 0 for me.  The open
syscall will return a negative number (-1) on error so 0 should be
valid.

This happens when one compiles a BOLT instrumented executable of
pyston (python implementation) and afterwards pip installs a package
which needs to be compiled and sets CFLAGS=-pipe.  I could not reduce
it down to a small testcase but I guess it one can trigger when
manually closing std{in, out, err}.  Everything seems to work
normally when disabling the assert for 0 in getBinaryPath() - I
decided to also modify the second case in readDescriptions() even
though I did not run into that one yet.

(cherry picked from FBD32409548)