Fix GCC6 -Wnull-dereference warnings.

Fix GCC6 -Wnull-dereference warnings.

asm_begin.c: In function ‘asm_begin’:
asm_begin.c:62:7: error: potential null pointer dereference [-Werror=null-dereference]
       __fsetlocking (result->out.file, FSETLOCKING_BYCALLER);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We set result to NULL just before this call in case of error.
Fixed by only calling __fsetlocking when result is not NULL.

strip.c: In function ‘handle_elf.constprop’:
strip.c:1270:31: error: null pointer dereference [-Werror=null-dereference]
       elf_assert ((versiondata->d_size / sizeof (Elf32_Word))
                    ~~~~~~~~~~~^~~

src/strip.c:597:37: note: in definition of macro ‘elf_assert’
 #define elf_assert(test) do { if (!(test)) goto illformed; } while (0)
                                     ^~~~

That is the wrong check, we want to check shndxdata, not versiondata here.

Signed-off-by: Mark Wielaard <mjw@redhat.com>