libbpf: Fix hypothetical STT_SECTION extern NULL deref case

Fix theoretical NULL dereference in linker when resolving *extern*
STT_SECTION symbol against not-yet-existing ELF section. Not sure if
it's possible in practice for valid ELF object files (this would require
embedded assembly manipulations, at which point BTF will be missing),
but fix the s/dst_sym/dst_sec/ typo guarding this condition anyways.

Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
Fixes: a46349227cd8 ("libbpf: Add linker extern resolution support for functions and global variables")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250220002821.834400-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>

src/linker.c

@@ -2163,7 +2163,7 @@ add_sym:
 
 	obj->sym_map[src_sym_idx] = dst_sym_idx;
 
-	if (sym_type == STT_SECTION && dst_sym) {
+	if (sym_type == STT_SECTION && dst_sec) {
 		dst_sec->sec_sym_idx = dst_sym_idx;
 		dst_sym->st_value = 0;
 	}