[flang][runtime] Signal format error when input field width is zero

[klausler]

Summary

A data edit descriptor for input may not have a zero field width.

flang/runtime/format-implementation.h

DataEdit FormatControl<CONTEXT>::GetNextDataEdit(
   if (edit.descriptor == 'A') { // width is optional for A[w]
     auto ch{PeekNext()};
     if (ch >= '0' && ch <= '9') {
       edit.width = GetIntField(context);
     }
   } else if (edit.descriptor != DataEdit::DefinedDerivedType) {
     edit.width = GetIntField(context);
   }
+  if constexpr (std::is_base_of_v<InputStatementState, CONTEXT>) {
+    if (edit.width.value_or(-1) == 0) {
+      ReportBadFormat(context, "Input field width is zero", start);
+    }
+  }
   if (edit.descriptor != DataEdit::DefinedDerivedType && PeekNext() == '.') {
     ++offset_;
     edit.digits = GetIntField(context);
     CharType ch{PeekNext()};
     if (ch == 'e' || ch == 'E' || ch == 'd' || ch == 'D') {
       ++offset_;
       edit.expoDigits = GetIntField(context);
     }