[flang][runtime] Don't loop in runtime if blank appears in BOZ input

[klausler]

Summary

The code for scanning BOZ input allows for blanks and tabs to appear,
but can hang if they do and the BOZ input value is not followed by
extra valid digits; the repositioning for the second sweep simply
needed to be done in units of character, not valid digits.

flang/runtime/edit-input.cpp

 
 namespace Fortran::runtime::io {
 
 template <int LOG2_BASE>
 static bool EditBOZInput(
     IoStatementState &io, const DataEdit &edit, void *n, std::size_t bytes) {
   std::optional<int> remaining;
   std::optional<char32_t> next{io.PrepareInput(edit, remaining)};
-  if (*next == '0') {
+  if (next.value_or('?') == '0') {
     do {
       next = io.NextInField(remaining, edit);
     } while (next && *next == '0');
   }
   // Count significant digits after any leading white space & zeroes
   int digits{0};
+  int chars{0};
   for (; next; next = io.NextInField(remaining, edit)) {
+    ++chars;
     char32_t ch{*next};
     if (ch == ' ' || ch == '\t') {
       continue;
     }
     if (ch >= '0' && ch <= '1') {
     } else if (LOG2_BASE >= 3 && ch >= '2' && ch <= '7') {
     } else if (LOG2_BASE >= 4 && ch >= '8' && ch <= '9') {
     } else if (LOG2_BASE >= 4 && ch >= 'A' && ch <= 'F') {
     } else if (LOG2_BASE >= 4 && ch >= 'a' && ch <= 'f') {
     } else {
       io.GetIoErrorHandler().SignalError(
           "Bad character '%lc' in B/O/Z input field", ch);
       return false;
     }
     ++digits;
   }
   auto significantBytes{static_cast<std::size_t>(digits * LOG2_BASE + 7) / 8};
   if (significantBytes > bytes) {
     io.GetIoErrorHandler().SignalError(IostatBOZInputOverflow,
         "B/O/Z input of %d digits overflows %zd-byte variable", digits, bytes);
     return false;
   }
   // Reset to start of significant digits
-  io.HandleRelativePosition(-digits);
+  io.HandleRelativePosition(-chars);
   remaining.reset();
   // Make a second pass now that the digit count is known
   std::memset(n, 0, bytes);
   int increment{isHostLittleEndian ? -1 : 1};
   auto *data{reinterpret_cast<unsigned char *>(n) +
       (isHostLittleEndian ? significantBytes - 1 : 0)};
   int shift{((digits - 1) * LOG2_BASE) & 7};
   if (shift + LOG2_BASE > 8) {