[ELF] Fix assertion failure

This patch fixes a regression introduced in
43fa021.

Fixes rui314#259

elf/passes.cc

@@ -523,13 +523,21 @@ void scan_rels(Context<E> &ctx) {
   });
 
   std::vector<Symbol<E> *> syms = flatten(vec);
+  ctx.symbol_aux.reserve(syms.size());
+
+  auto add_aux = [&](Symbol<E> *sym) {
+    if (sym->aux_idx == -1) {
+      i64 sz = ctx.symbol_aux.size();
+      sym->aux_idx = sz;
+      ctx.symbol_aux.resize(sz + 1);
+    }
+  };
 
-  ctx.symbol_aux.resize(syms.size());
-  for (i64 i = 0; i < syms.size(); i++)
-    syms[i]->aux_idx = i;
 
   // Assign offsets in additional tables for each dynamic symbol.
   for (Symbol<E> *sym : syms) {
+    add_aux(sym);
+
     if (sym->is_imported || sym->is_exported)
       ctx.dynsym->add_symbol(ctx, sym);
 
@@ -583,6 +591,7 @@ void scan_rels(Context<E> &ctx) {
       // Aliases of this symbol are also copied so that they will be
       // resolved to the same address at runtime.
       for (Symbol<E> *alias : file->find_aliases(sym)) {
+        add_aux(alias);
         alias->is_imported = true;
         alias->is_exported = true;
         alias->has_copyrel = true;

test/elf/copyrel.sh

@@ -20,15 +20,14 @@ int main() {
 }
 EOF
 
-cat <<EOF | cc -o "$t"/b.o -c -x assembler -
-  .globl foo, bar
-  .data;
-foo:
-bar:
-  .long 42
+cat <<EOF | cc -fPIC -o "$t"/b.o -c -xc -
+int foo = 42;
+extern int bar __attribute__((alias("foo")));
+extern int baz __attribute__((alias("foo")));
 EOF
 
-clang -fuse-ld="$mold" -no-pie -o "$t"/exe "$t"/a.o "$t"/b.o
+clang -fuse-ld="$mold" -shared -o "$t"/c.so "$t"/b.o
+clang -fuse-ld="$mold" -no-pie -o "$t"/exe "$t"/a.o "$t"/c.so
 "$t"/exe | grep -q '42 42 1'
 
 echo OK