Avoid SEGFAULT if a requested symbol section is absent.

Patch by Igor Kudrin!

llvm-svn: 248194
diff --git a/llvm/test/Object/readobj-absent.test b/llvm/test/Object/readobj-absent.test
new file mode 100644
index 0000000..0968f3c
--- /dev/null
+++ b/llvm/test/Object/readobj-absent.test
@@ -0,0 +1,2 @@
+// Don't crash if required information is absent
+RUN: llvm-readobj -dyn-symbols %p/Inputs/trivial-object-test.elf-x86-64