sparc64: fix build with older gccs

Fix errors missed in 2065061ede22d401aae2ce995c3af54db9d28639:
 CC    sparc64-softmmu/helper.o
cc1: warnings being treated as errors
/src/qemu/target-sparc/helper.c: In function 'get_physical_address':
/src/qemu/target-sparc/helper.c:426: warning: 'context' may be used uninitialized in this function
/src/qemu/target-sparc/helper.c:426: note: 'context' was declared here

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
