objtool: Do not retrieve data from empty sections

Binutils 2.29-9 in Debian return an error when elf_getdata is invoked
on empty section (.note.GNU-stack in all kernel files), causing
immediate failure of kernel build with:

  elf_getdata: can't manipulate null section

As nothing is done with sections that have zero size, just do not
retrieve their data at all.
Signed-off-by: Petr Vandrovec <petr@vandrovec.name>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/2ce30a44349065b70d0f00e71e286dc0cbe745e6.1505459652.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/elf.c

@@ -175,19 +175,20 @@ static int read_sections(struct elf *elf)
 			return -1;
 		}
 
+		if (sec->sh.sh_size != 0) {
 			sec->data = elf_getdata(s, NULL);
 			if (!sec->data) {
 				WARN_ELF("elf_getdata");
 				return -1;
 			}
-
 			if (sec->data->d_off != 0 ||
 			    sec->data->d_size != sec->sh.sh_size) {
-			WARN("unexpected data attributes for %s", sec->name);
+				WARN("unexpected data attributes for %s",
+				     sec->name);
 				return -1;
 			}
-
-		sec->len = sec->data->d_size;
+		}
+		sec->len = sec->sh.sh_size;
 	}
 
 	/* sanity check, one more call to elf_nextscn() should return NULL */