readelf: Don't print average number of tests when no tests are done

readelf: Don't print average number of tests when no tests are done

If the symbol hash table only contains lenght zero chains, no lookup
tests need to be done and eu-readelf -I would print out bogus numbers
for the number of tests that were successful/unsuccessful.

e.g. for an "empty" program like
  int main() {}
eu-readelf -I would print:

Histogram for bucket list length in section [ 5] '.gnu.hash' (total of 1 bucket):
 Addr: 0x00000000004003c0  Offset: 0x0003c0  Link to section: [ 6] '.dynsym'
 Symbol Bias: 1
 Bitmask Size: 8 bytes  0% bits set  2nd hash shift: 0
 Length  Number  % of total  Coverage
      0       1      100.0%
 Average number of tests:   successful lookup: -nan
			  unsuccessful lookup: 0.000000

Only print out the Average number of tests when there were actual
tests to do.

Signed-off-by: Mark Wielaard <mark@klomp.org>