Don't try to parse .dynamic section of type NOBITS

Otherwise, patchelf segfaults when it encounters DT_NEEDED in the read
garbage. Corresponding backtrace is:

    #0  0x00007ffff7c275f7 in __strlen_avx2 () from /nix/store/cvr0kjg2q7z2wwhjblx6c73rv422k8cm-glibc-2.33-47/lib/libc.so.6
    #1  0x00007ffff7f2d448 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) () from /nix/store/lg104nh0szci8slz5z6494m457jm5y3p-gcc-10.3.0-lib/lib/libstdc++.so.6
    #2  0x000000000040fe0f in ElfFile<Elf64_Ehdr, Elf64_Phdr, Elf64_Shdr, unsigned long, unsigned long, Elf64_Dyn, Elf64_Sym, Elf64_Verneed, unsigned short>::modifyRPath (this=0x7fffffffbaa0,
        op=ElfFile<Elf64_Ehdr, Elf64_Phdr, Elf64_Shdr, unsigned long, unsigned long, Elf64_Dyn, Elf64_Sym, Elf64_Verneed, unsigned short>::rpPrint, allowedRpathPrefixes=std::vector of length 0, capacity 0, newRPath="") at patchelf.cc:1351
    #3  0x00000000004061c3 in patchElf2<ElfFile<Elf64_Ehdr, Elf64_Phdr, Elf64_Shdr, unsigned long, unsigned long, Elf64_Dyn, Elf64_Sym, Elf64_Verneed, unsigned short> > (elfFile=..., fileContents=std::shared_ptr<std::vector<unsigned char, std::allocator<unsigned char> >> (use count 3, weak count 0) = {...},
        fileName="libsystemd.debug") at patchelf.cc:1805
    #4  0x0000000000404774 in patchElf () at patchelf.cc:1848
    #5  0x000000000040551c in mainWrapped (argc=3, argv=0x7fffffffc148) at patchelf.cc:2003
    #6  0x0000000000405913 in main (argc=3, argv=0x7fffffffc148) at patchelf.cc:2011

NOBIT sections are included in the section headers table but occupy no
actual space in the file. .dynamic sections of this types are created,
for example, by `strip --only-keep-debug`.

I'm not sure whether calling error() would be more appropriate than
ignoring this situation with debug/return. I chose ignoring it,
because error() caused autoPatchelfHook to fail with my package. Also
the rest of modifyRPath method simply calls debug/return in similar
situations.

src/patchelf.cc

@@ -1330,6 +1330,11 @@ void ElfFile<ElfFileParamNames>::modifyRPath(RPathOp op,
 {
     auto shdrDynamic = findSection(".dynamic");
 
+    if (rdi(shdrDynamic.sh_type) == SHT_NOBITS) {
+            debug("no dynamic section\n");
+            return;
+    }
+
     /* !!! We assume that the virtual address in the DT_STRTAB entry
        of the dynamic section corresponds to the .dynstr section. */
     auto shdrDynStr = findSection(".dynstr");