linux: Fix segfault when there is no section header in ELF file

Some vendor library doesn't contain the section header. It causes
segmentation fault in `FindElfClassSection`.

> e_shoff:
> This member holds the section header table's file offset in bytes.
> If the file has no section header table this member holds zero.

Change-Id: Id98d6ff3bd16af4541deb5a55a8fad2fa74eda23
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2354427
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/linux/elfutils.cc

diff --git a/src/common/linux/elfutils.cc b/src/common/linux/elfutils.cc
index ce9e9c1..aa95357 100644
--- a/src/common/linux/elfutils.cc
+++ b/src/common/linux/elfutils.cc
@@ -57,6 +57,12 @@
   const Ehdr* elf_header = reinterpret_cast<const Ehdr*>(elf_base);
   assert(elf_header->e_ident[EI_CLASS] == ElfClass::kClass);
 
+  if (elf_header->e_shoff == 0) {
+    *section_start = NULL;
+    *section_size = 0;
+    return;
+  }
+
   const Shdr* sections =
     GetOffset<ElfClass, Shdr>(elf_header, elf_header->e_shoff);
   const Shdr* section_names = sections + elf_header->e_shstrndx;