gobuffer: Do not crash on object without any public symbols

pahole -J .tmp_linux.btf during Linux build process always crashes
on my system.

Problem is that when gobuffer is initialized via gobuffer__init(),
it is in state where 'index' (AKA its size) is set to 1, but
'entries' is NULL.

State corrects itself if 'gobuffer__add()' is invoked, as that
will allocate buffer (even if added len == 0).  But if __add()
is never invoked because only anonymous symbols are present,
one ends up with gobuffer that crashes gobuffer__copy.

Instead of allocating single-byte buffer always I opted for
checking if gobuffer entries is NULL before use in copy and
compress - gobuffer__init() would need prototype change to
report malloc failures, and it seems unnecessary to allocate
memory always - even if file does not have any symbols after
all.

(gdb) bt
 #0  __memmove_sse2_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:287
 acmel#1  0x00007f2f2c1ec2ee in btf_elf__encode (btfe=0x5654e31e2e30, flags=flags@entry=0 '\000') at libbtf.c:750
 acmel#2  0x00007f2f2c1e9af0 in btf_encoder__encode () at btf_encoder.c:164
 acmel#3  0x00005654e2407599 in main (argc=3, argv=0x7ffcd8783f18) at pahole.c:1344
(gdb) frame 1
 acmel#1  0x00007f2f2c1ec2ee in btf_elf__encode (btfe=0x5654e31e2e30, flags=flags@entry=0 '\000') at libbtf.c:750
750             gobuffer__copy(btfe->strings, btf_elf__nohdr_data(btfe) + hdr->str_off);
(gdb) print btfe->strings
$1 = (struct gobuffer *) 0x5654e31db2c8
(gdb) print *btfe->strings
$2 = {entries = 0x0, nr_entries = 0, index = 1, allocated_size = 0}
(gdb) print btfe->types
$3 = {entries = 0x5654e31e2ef0 "", nr_entries = 1, index = 16, allocated_size = 8192}
(gdb) x /16bx btfe->types.entries
0x5654e31e2ef0: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x01
0x5654e31e2ef8: 0x04    0x00    0x00    0x00    0x20    0x00    0x00    0x00

Signed-off-by: Petr Vandrovec <petr@vmware.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

gobuffer.c

@@ -91,7 +91,12 @@ int gobuffer__add(struct gobuffer *gb, const void *s, unsigned int len)
 
 void gobuffer__copy(const struct gobuffer *gb, void *dest)
 {
-	memcpy(dest, gb->entries, gobuffer__size(gb));
+        if (gb->entries) {
+		memcpy(dest, gb->entries, gobuffer__size(gb));
+	} else {
+		/* gobuffer__size will be 0 or 1. */
+		memcpy(dest, "", gobuffer__size(gb));
+	}
 }
 
 const void *gobuffer__compress(struct gobuffer *gb, unsigned int *size)
@@ -101,7 +106,7 @@ const void *gobuffer__compress(struct gobuffer *gb, unsigned int *size)
 		.zfree	  = Z_NULL,
 		.opaque	  = Z_NULL,
 		.avail_in = gobuffer__size(gb),
-		.next_in  = (Bytef *)gobuffer__entries(gb),
+		.next_in  = (Bytef *)(gobuffer__entries(gb) ? : ""),
 	};
 	void *bf = NULL;
 	unsigned int bf_size = 0;