[LLD][ELF] - Do not crash when parsing the -defsym option from a error state.

When we are in a error state, script parser will not parse the -defsym
expression and hence will not tokenize it. Then ScriptLexer::Pos will be 0
and LLD will assert and crash here:

MemoryBufferRef ScriptLexer::getCurrentMB() {
  assert(!MBs.empty() && Pos > 0); // Bang !

Solution - stop parsing the defsym in a error state. That is consistent
with the regular case (when we parse the linker script).

llvm-svn: 347549

lld/ELF/ScriptParser.cpp

@@ -271,6 +271,8 @@ void ScriptParser::readLinkerScript() {
 }
 
 void ScriptParser::readDefsym(StringRef Name) {
+  if (errorCount())
+    return;
   Expr E = readExpr();
   if (!atEOF())
     setError("EOF expected, but got " + next());

lld/test/ELF/defsym.s

@@ -9,6 +9,12 @@
 # RUN: llvm-readobj -t -s %t2 | FileCheck %s
 # RUN: llvm-objdump -d -print-imm-hex %t2 | FileCheck %s --check-prefix=USE
 
+## Check we are reporting the error correctly and don't crash
+## when handling the second --defsym.
+# RUN: not ld.lld -o %t2 %t.o --defsym ERR+ \
+#        --defsym foo2=foo1 2>&1 | FileCheck %s --check-prefix=ERR
+# ERR: error: -defsym: syntax error: ERR+
+
 # CHECK:      Symbol {
 # CHECK:        Name: foo1
 # CHECK-NEXT:   Value: 0x123