Suppress may be used uninitialized warning

It seems that not every GCC has `-Wno-maybe-uninitialized` support. When building
with GCC version is 4.6.1 there was a warning:

    libudis86/decode.c: In function ‘ud_decode’:
    libudis86/decode.c:284:34: warning: ‘curr’ may be used uninitialized in this function [-Wuninitialized]
    libudis86/decode.c:231:11: note: ‘curr’ was declared here
    libudis86/decode.c: At top level:
    cc1: warning: unrecognized command line option "-Wno-maybe-uninitialized" [enabled by default]

Remove this by initializing the variable.

Thanks to @alexandernst for testing.