[flang][runtime] Fix input of NAN(...) on non-fast path

[klausler]

Summary

The closing parenthesis needs to be consumed when a NaN
with parenthesized (ignored) information is read on the
real input path that preprocesses input characters before
passing them to the decimal-to-binary converter.

flang/runtime/edit-input.cpp

     if (next && *next == '(') { // NaN(...)
       Put('(');
       int depth{1};
-      do {
+      while (true) {
         next = io.NextInField(remaining, edit);
-        if (!next) {
+        if (depth == 0) {
           break;
+        } else if (!next) {
+          return 0; // error
         } else if (*next == '(') {
           ++depth;
         } else if (*next == ')') {
           --depth;
         }
         Put(*next);
-      } while (depth > 0);
+      }
     }
     exponent = 0;
   } else if (first == decimal || (first >= '0' && first <= '9') ||