COFF: Add an assertion. NFC.

r239458 changed callee side of this function, so Live can never be
true when this function is called.

llvm-svn: 239705

lld/COFF/Chunks.cpp

@@ -64,8 +64,7 @@ void SectionChunk::writeTo(uint8_t *Buf) {
 }
 
 void SectionChunk::mark() {
-  if (Live)
-    return;
+  assert(!Live);
   Live = true;
 
   // Mark all symbols listed in the relocation table for this section.