[llvm-readobj] - Don`t crash when --section-symbols is requested for an object w/o .symtab.

[grimar]

Summary

llvm-readobj crashes when -S --section-symbols is used
on an object that has no symbol table.

The patch fixes it.

Depends on D86462.

[MaskRay]

Looks great!

[jhenderson]

LGTM too.

llvm/test/tools/llvm-readobj/ELF/sections-ext.test

- Name: .rela.text
     Relocations:
       - Type: R_X86_64_NONE
   - Name: .bss
     Type: SHT_NOBITS
     Size: 0x1000
 Symbols:
   - Name:    foo
     Section: .text
+
+## Check the output for an arbitrary section when --section-symbols is requested,
+## but there is no symbol table in the object.
+# RUN: yaml2obj --docnum=2 %s -o %t2
+# RUN: llvm-readobj -S --section-symbols %t2 2>&1 | \
+# RUN:   FileCheck %s --implicit-check-not=warning: --check-prefix=NOSYMTAB
+
+# NOSYMTAB:      Section {
+# NOSYMTAB:        Index: 1
+# NOSYMTAB-NEXT:   Name: .foo
+# NOSYMTAB:        Symbols [
+# NOSYMTAB-NEXT:   ]
+# NOSYMTAB-NEXT: }
+# NOSYMTAB-NEXT: Section {
+# NOSYMTAB-NEXT:   Index: 2
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Sections:
+  - Name: .foo
+    Type: SHT_PROGBITS

llvm/tools/llvm-readobj/ELFDumper.cpp

for (const Elf_Shdr &Sec : cantFail(Obj->sections())) {
 
     if (opts::SectionRelocations) {
       ListScope D(W, "Relocations");
       this->printRelocationsHelper(Obj, Sec);
     }
 
     if (opts::SectionSymbols) {
       ListScope D(W, "Symbols");
-      const Elf_Shdr *Symtab = this->dumper()->getDotSymtabSec();
-      StringRef StrTable =
-          unwrapOrError(this->FileName, Obj->getStringTableForSymtab(*Symtab));
+      if (const Elf_Shdr *Symtab = this->dumper()->getDotSymtabSec()) {
+        StringRef StrTable = unwrapOrError(
+            this->FileName, Obj->getStringTableForSymtab(*Symtab));
 
         for (const Elf_Sym &Sym :
              unwrapOrError(this->FileName, Obj->symbols(Symtab))) {
           const Elf_Shdr *SymSec = unwrapOrError(
               this->FileName,
               Obj->getSection(&Sym, Symtab, this->dumper()->getShndxTable()));
           if (SymSec == &Sec)
             printSymbol(
                 Obj, &Sym,
                 unwrapOrError(this->FileName, Obj->symbols(Symtab)).begin(),
                 StrTable, false, false);
         }
       }
+    }
 
     if (opts::SectionData && Sec.sh_type != ELF::SHT_NOBITS) {
       ArrayRef<uint8_t> Data =
           unwrapOrError(this->FileName, Obj->getSectionContents(&Sec));
       W.printBinaryBlock(
           "SectionData",
           StringRef(reinterpret_cast<const char *>(Data.data()), Data.size()));
     }