[sh] Fix a "may be unused" warning on mips-gcc

mips-gcc for mips32 was complaining that c was potentially used before
being set. Setting it to 0 before calling fdgetsc() looks like the right
thing to do in this instance; there's an explicit check for c == 0 later
on.

Tested: mips-gcc mips32 build, running /bin/sh on mips32