Check a symbol variant exists before creation in AsmParser.

[BrandonTJones]

Summary

Check that the variant of the symbol is not empty before attempting to to treat it as the symbol name.

llvm/lib/MC/MCParser/AsmParser.cpp

case AsmToken::Identifier: {
     if (SymbolName.empty())
       return Error(getLexer().getLoc(), "expected a symbol reference");
 
     MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
 
     // Lookup the symbol variant if used.
     if (!Split.second.empty()) {
       Variant = MCSymbolRefExpr::getVariantKindForName(Split.second);
-      if (Variant != MCSymbolRefExpr::VK_Invalid) {
+      if (Variant != MCSymbolRefExpr::VK_Invalid && !Split.first.empty()) {
         SymbolName = Split.first;
       } else if (MAI.doesAllowAtInName() && !MAI.useParensForSymbolVariant()) {
         Variant = MCSymbolRefExpr::VK_None;
       } else {
         return Error(SMLoc::getFromPointer(Split.second.begin()),
                      "invalid variant '" + Split.second + "'");
       }
     }