[LLD][ELF] - Fix crash when using empty --defsym.

[grimar]

Summary

We have a crash issue when handling the empty -defsym.

For parsing this option we are using ScriptParser class which is used
generally for reading the linker script. For empty defsym case, we
pass the empty memory buffer and crash in the place removed in https://reviews.llvm.org/rL336436.

But reverting of the above patch would not help here (we would still crash but a bit later). And
even after fixing the crash we would report something like
"lld.exe: error: -defsym:1: unexpected EOF"
It is probably not the appropriate message because mentions EOF.

I think the issue should be handled on a higher level like this patch does.
So we do not want to pass the empty memory buffer first of all I believe.

[ruiu]

LGTM

ELF/Driver.cpp

1030

Please include the option value itself as well:

"-defsym: syntax error: " + Arg->getValue()

ELF/Driver.cpp

case OPT_library:
       break;
     case OPT_INPUT:
       addFile(Arg->getValue(), /*WithLOption=*/false);
       break;
     case OPT_defsym: {
       StringRef From;
       StringRef To;
       std::tie(From, To) = StringRef(Arg->getValue()).split('=');
+      if (From.empty() || To.empty())
+        error("-defsym: syntax error");
+      else
         readDefsym(From, MemoryBufferRef(To, "-defsym"));
       break;
     }
     case OPT_script:
       if (Optional<std::string> Path = searchScript(Arg->getValue())) {
         if (Optional<MemoryBufferRef> MB = readFile(*Path))
           readLinkerScript(*MB);
         break;
       }

test/ELF/defsym.s

 # EXPR-NEXT:   Value: 0x128
 # EXPR-NEXT:   Size:
 # EXPR-NEXT:   Binding: Global
 # EXPR-NEXT:   Type:
 # EXPR-NEXT:   Other:
 # EXPR-NEXT:   Section: Absolute
 # EXPR-NEXT: }
 
-# RUN: not ld.lld -o %t %t.o --defsym=foo2=und 2>&1 | FileCheck %s -check-prefix=ERR
-# ERR: error: -defsym:1: symbol not found: und
+# RUN: not ld.lld -o %t %t.o --defsym=foo2=und 2>&1 | FileCheck %s -check-prefix=ERR1
+# ERR1: error: -defsym:1: symbol not found: und
+
+# RUN: not ld.lld -o %t %t.o --defsym= 2>&1 | FileCheck %s -check-prefix=ERR2
+# RUN: not ld.lld -o %t %t.o --defsym=foo 2>&1 | FileCheck %s -check-prefix=ERR2
+# ERR2: error: -defsym: syntax error
 
 .globl foo1
  foo1 = 0x123
 
 .global _start
 _start:
   movl $foo2, %edx