libblkid: ensure uninitialized variable is not used [clang]

libblkid: ensure uninitialized variable is not used [clang]

Following condition can be true if minix file system is corrupt, and versio
number is found to be greater than 3.  It is fair to say described scenario
is unlikely.

libblkid/src/superblocks/minix.c:107:13: warning: variable 'zone_size' is
used uninitialized whenever
      'if' condition is false [-Wsometimes-uninitialized]
        } else if (version == 3) {
                   ^~~~~~~~~~~~
libblkid/src/superblocks/minix.c:121:6: note: uninitialized use occurs here
        if (zone_size != 0 || ninodes == 0 || ninodes == UINT32_MAX)
            ^~~~~~~~~

Signed-off-by: Sami Kerola <kerolasa@iki.fi>