[llvm-objdump] - Stop reporting bogus section IDs.

Imagine we have a file with few sections, and one of them is .foo
with index N != 0.

Problem is that when llvm-objdump is given a -section=.foo parameter
it lists .foo as a section at index 0. That makes impossible to write
test cases which needs to find the index of the particular section,
while ignoring dumping of others.

The patch fixes that.

Differential revision: https://reviews.llvm.org/D49372

llvm-svn: 337361