[ELF] - Don't crash on broken MEMORY declaration.

[grimar]

Summary

LLD crashes with broken scripts shown in testcase,
because fails to read memory regon name and accesses
MemoryRegions's element which is nullptr.
Patch fixes it.

[grimar]

• Added one more broken case.

[ruiu]

LGTM

ELF/ScriptParser.cpp

if (Tok == "DATA_SEGMENT_RELRO_END") {
     return [=] { return alignTo(Script->getDot(), E().getValue()); };
   }
   if (Tok == "DEFINED") {
     StringRef Name = readParenLiteral();
     return [=] { return Symtab->find(Name) ? 1 : 0; };
   }
   if (Tok == "LENGTH") {
     StringRef Name = readParenLiteral();
-    if (Script->MemoryRegions.count(Name) == 0)
+    if (Script->MemoryRegions.count(Name) == 0) {
       setError("memory region not defined: " + Name);
+      return [=] { return 0; };
+    }
     return [=] { return Script->MemoryRegions[Name]->Length; };
   }
   if (Tok == "LOADADDR") {
     StringRef Name = readParenLiteral();
     OutputSection *Cmd = Script->getOrCreateOutputSection(Name);
     return [=] {
       checkIfExists(Cmd, Location);
       return Cmd->getLMA();
     };
   }
   if (Tok == "ORIGIN") {
     StringRef Name = readParenLiteral();
-    if (Script->MemoryRegions.count(Name) == 0)
+    if (Script->MemoryRegions.count(Name) == 0) {
       setError("memory region not defined: " + Name);
+      return [=] { return 0; };
+    }
     return [=] { return Script->MemoryRegions[Name]->Origin; };
   }
   if (Tok == "SEGMENT_START") {
     expect("(");
     skip();
     expect(",");
     Expr E = readExpr();
     expect(")");

test/ELF/linkerscript/broken-memory-declaration.s

+# REQUIRES: x86
+
+## Check we do not crash.
+
+# RUN: echo "MEMORY { FLASH (rx) : ORIGIN = 0x1000< LENGTH" > %t.script
+# RUN: not ld.lld -o %t --script %t.script 2>&1 | FileCheck %s
+# CHECK: unexpected EOF
+
+# RUN: echo "MEMORY { FLASH (rx) : ORIGIN = 0x1000< ORIGIN" > %t.script
+# RUN: not ld.lld -o %t --script %t.script 2>&1 | FileCheck %s