[libunwind] Don't abort if encoutering invalid .eh_frame_hdr

Recent Linux kernel release has introduced a bug as part of the ORC
rollout where the vDSO has a valid .eh_frame section, but it's missing
the .eh_frame_hdr section and GNU_EH_FRAME segment has zero size. This
causes libunwind to abort which breaks programs that use libunwind.

The other unwinder implementation (libgcc, non-gnu) instead silently
bail out unless being compiled as debug. This change modifies libunwind
to use the same strategy.

Differential Revision: https://reviews.llvm.org/D57081

llvm-svn: 352016
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 8c84e00c71f341d1e4bf0331f1a4b78879fe7f14
diff --git a/src/AddressSpace.hpp b/src/AddressSpace.hpp
index 524dbaa..29a821c 100644
--- a/src/AddressSpace.hpp
+++ b/src/AddressSpace.hpp
@@ -535,11 +535,11 @@
 #endif
             cbdata->sects->dwarf_index_section = eh_frame_hdr_start;
             cbdata->sects->dwarf_index_section_length = phdr->p_memsz;
-            EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
+            found_hdr = EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
                 *cbdata->addressSpace, eh_frame_hdr_start, phdr->p_memsz,
                 hdrInfo);
-            cbdata->sects->dwarf_section = hdrInfo.eh_frame_ptr;
-            found_hdr = true;
+            if (found_hdr)
+              cbdata->sects->dwarf_section = hdrInfo.eh_frame_ptr;
           }
         }