[flang][runtime] Fix BACKSPACE over an empty record

[klausler]

Summary

The implementation of BACKSPACE on a variable-length sequential formatted
file has a bug that prevents it from working on an empty record.

[vzakhari]

Thank you!

flang/runtime/unit.cpp

if (header != *recordLength) {
     handler.SignalError(IostatBadUnformattedRecord);
     return;
   }
 }
 
 // There's no portable memrchr(), unfortunately, and strrchr() would
 // fail on a record with a NUL, so we have to do it the hard way.
 static const char *FindLastNewline(const char *str, std::size_t length) {
-  for (const char *p{str + length}; p-- > str;) {
+  for (const char *p{str + length}; p >= str; p--) {
     if (*p == '\n') {
       return p;
     }
   }
   return nullptr;
 }
 
 void ExternalFileUnit::BackspaceVariableFormattedRecord(