[flang][runtime] When NAMELIST input hits EOF, signal END, not an error

[klausler]

Summary

NAMELIST input processing in the runtime support library treats an
end-of-file found while searching for the initial '&' character
as an error condition, but it really should be distinguishable.
Call SignalEnd() rather than SignalError().

[jeanPerier]

LGTM

flang/runtime/namelist.cpp

while (true) {
     while (next && *next != '&') {
       // Extension: comment lines without ! before namelist groups
       if (!io.AdvanceRecord()) {
         next.reset();
       } else {
         next = io.GetNextNonBlank(byteCount);
       }
     }
-    if (!next || *next != '&') {
+    if (!next) {
+      handler.SignalEnd();
+      return false;
+    }
+    if (*next != '&') {
       handler.SignalError(
           "NAMELIST input group does not begin with '&' (at '%lc')", *next);
       return false;
     }
     io.HandleRelativePosition(byteCount);
     if (!GetLowerCaseName(io, name, sizeof name)) {
       handler.SignalError("NAMELIST input group has no name");
       return false;