[ELF] Don't cause assertion failure if --dynamic-list or --version-script takes an empty file

Fixes PR46184
Report line 1 of the last memory buffer.

lld/ELF/ScriptLexer.cpp

@@ -52,6 +52,8 @@ StringRef ScriptLexer::getLine() {
 
 // Returns 1-based line number of the current token.
 size_t ScriptLexer::getLineNumber() {
+  if (pos == 0)
+    return 1;
   StringRef s = getCurrentMB().getBuffer();
   StringRef tok = tokens[pos - 1];
   return s.substr(0, tok.data() - s.data()).count('\n') + 1;
@@ -292,7 +294,9 @@ static bool encloses(StringRef s, StringRef t) {
 
 MemoryBufferRef ScriptLexer::getCurrentMB() {
   // Find input buffer containing the current token.
-  assert(!mbs.empty() && pos > 0);
+  assert(!mbs.empty());
+  if (pos == 0)
+    return mbs.back();
   for (MemoryBufferRef mb : mbs)
     if (encloses(mb.getBuffer(), tokens[pos - 1]))
       return mb;

lld/test/ELF/invalid-dynamic-list.test

@@ -9,6 +9,10 @@
 
 # RUN: mkdir -p %t.dir
 
+# RUN: echo > %tempty.list
+# RUN: not ld.lld --dynamic-list %tempty.list 2>&1 | FileCheck --check-prefix=EMPTY %s
+# EMPTY: error: {{.*}}.list:1: unexpected EOF
+
 # RUN: echo foobar > %t1
 # RUN: not ld.lld --dynamic-list %t1 2>&1 | FileCheck -check-prefix=ERR1 %s
 # ERR1: {{.*}}:1: { expected, but got foobar

lld/test/ELF/version-script-err.s

@@ -8,3 +8,8 @@
 // RUN: not ld.lld --version-script %terr1.script -shared %t.o -o /dev/null 2>&1 | \
 // RUN:   FileCheck -check-prefix=ERR1 %s
 // ERR1: {{.*}}:1: unclosed quote
+
+// RUN: echo > %tempty.ver
+// RUN: not ld.lld --version-script %tempty.ver 2>&1 | \
+// RUN:   FileCheck --check-prefix=ERR2 %s
+// ERR2: error: {{.*}}.ver:1: unexpected EOF