[mlir] NFC - Fix OperationSupport.cpp::findNamedAttr

[nicolasvasilache]

Summary

When NamedAttrList::get is called against a StringRef and the
entry is not present, the Identifier::operator== crashes.
Interestingly there seems to be no use of the NamedAttrList::get(StringRef) in
the codebase so far. A subsequent commit will introduce such a use.

mlir/lib/IR/OperationSupport.cpp

 static auto *findAttr(SmallVectorImpl<NamedAttribute> &attrs, T name,
                       bool sorted) {
   if (!sorted) {
     return llvm::find_if(
         attrs, [name](NamedAttribute attr) { return attr.first == name; });
   }
 
   auto *it = llvm::lower_bound(attrs, name);
-  if (it->first != name)
+  if (it == attrs.end() || it->first != name)
     return attrs.end();
   return it;
 }
 
 /// Return the specified attribute if present, null otherwise.
 Attribute NamedAttrList::get(StringRef name) const {
   auto *it = findAttr(attrs, name, isSorted());
   return it != attrs.end() ? it->second : nullptr;