[llvm-readelf] - Do not crash when dumping the dynamic symbol table when its sh_entzize == 0.

[grimar]

Summary

We have a division by zero crash currently when
the sh_entzize of the dynamic symbol table is 0.

[jhenderson]

LGTM.

llvm/test/tools/llvm-readobj/ELF/dyn-symbols.test

 ## An arbitrary valid symbol to document we report an error before dumping it.
   - StName:  0x1
 ProgramHeaders:
   - Type:  PT_LOAD
     Flags: [ PF_R ]
     Sections:
       - Section: .dynstr
       - Section: .dynamic
+
+## Check we report a warning when the entry size of the dynamic symbol table is zero.
+# RUN: yaml2obj %s --docnum=14 -o %t15
+# RUN: llvm-readobj --dyn-symbols %t15 2>&1 | FileCheck %s -DFILE=%t15 --check-prefix=DYNSYM-ZERO-ENTSIZE-LLVM
+# RUN: llvm-readelf --dyn-symbols %t15 2>&1 | \
+# RUN:   FileCheck %s -DFILE=%t15 --check-prefix=DYNSYM-ZERO-ENTSIZE-GNU --implicit-check-not="Symbol table"
+
+# DYNSYM-ZERO-ENTSIZE-LLVM:      DynamicSymbols [
+# DYNSYM-ZERO-ENTSIZE-LLVM-NEXT:  warning: '[[FILE]]': section with index 1 has invalid size (0x10) or entry size (0x0)
+# DYNSYM-ZERO-ENTSIZE-LLVM-NEXT: ]
+
+# DYNSYM-ZERO-ENTSIZE-GNU:  warning: '[[FILE]]': section with index 1 has invalid size (0x10) or entry size (0x0)
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_386
+Sections:
+  - Name:    .dynsym
+    Type:    SHT_DYNSYM
+    EntSize: 0x0

llvm/tools/llvm-readobj/ELFDumper.cpp

void ELFDumper<ELFT>::printSymbolsHelper(bool IsDynamic) const {
   StringRef StrTable, SymtabName;
   size_t Entries = 0;
   Elf_Sym_Range Syms(nullptr, nullptr);
   const ELFFile<ELFT> *Obj = ObjF->getELFFile();
   if (IsDynamic) {
     StrTable = DynamicStringTable;
     Syms = dynamic_symbols();
     SymtabName = DynSymtabName;
-    if (DynSymRegion)
-      Entries = DynSymRegion->Size / DynSymRegion->EntSize;
+    Entries = Syms.size();
   } else {
     if (!DotSymtabSec)
       return;
     StrTable = unwrapOrError(ObjF->getFileName(),
                              Obj->getStringTableForSymtab(*DotSymtabSec));
     Syms = unwrapOrError(ObjF->getFileName(), Obj->symbols(DotSymtabSec));
     SymtabName =
         unwrapOrError(ObjF->getFileName(), Obj->getSectionName(DotSymtabSec));