[flang] runtime: fix formatted real input regression w/ spaces

[klausler]

Summary

Blank input fields must be interpreted as zero, including the case of
virutal space characters generated from record padding at the end of
an input record. This stopped working sometime in the past few months
for real input (not sure when); here's a fix.

This bug was breaking FCVS test fm111.

[klausler]

Renaud, if you aren't set up to review Phabricator changes, please let me know.

flang/runtime/edit-input.cpp

auto Put{[&](char ch) -> void {
     if (got < bufferSize) {
       buffer[got] = ch;
     }
     ++got;
   }};
   if (ScanNumericPrefix(io, edit, next, remaining)) {
     Put('-');
   }
-  if (!next) { // empty field means zero
+  if (next.value_or(' ') == ' ') { // empty/blank field means zero
+    remaining.reset();
     Put('0');
     return got;
   }
   char32_t decimal = edit.modes.editingFlags & decimalComma ? ',' : '.';
   char32_t first{*next >= 'a' && *next <= 'z' ? *next + 'A' - 'a' : *next};
   if (first == 'N' || first == 'I') {
     // NaN or infinity - convert to upper case
     // Subtle: a blank field of digits could be followed by 'E' or 'D',