Fix crash in llvm-objdump with -macho -objc-meta-data that was trying dump a non-existent section.

Showed up in running on a large binary with the missing section.  I could create a fake
test case if anyone really wants but the fix is pretty obvious.

rdar://25837034


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267037 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-objdump/MachODump.cpp

@@ -5134,6 +5134,9 @@ static void print_image_info32(SectionRef S, struct DisassembleInfo *info) {
   struct objc_image_info32 o;
   const char *r;
 
+  if (S == SectionRef())
+    return;
+
   StringRef SectName;
   S.getName(SectName);
   DataRefImpl Ref = S.getRawDataRefImpl();