[flang][runtime] Better error message for mis-ASSIGN'ed FORMAT statement

[klausler]

When an I/O data transfer statement uses an ASSIGN'ed FORMAT that
has not been ASSIGN'ed to a FORMAT statement, the runtime receives
a zero-length format string. Distinguish this case from the general
error message about missing parentheses.

[clementval]

LGTM

flang/runtime/format.h

   CharType GetNextChar(IoErrorHandler &handler) {
     SkipBlanks();
     if (offset_ >= formatLength_) {
-      handler.SignalError(
-          IostatErrorInFormat, "FORMAT missing at least one ')'");
+      if (formatLength_ == 0) {
+        handler.SignalError(
+            IostatErrorInFormat, "Empty or badly assigned FORMAT");
+      } else {
+        handler.SignalError(
+            IostatErrorInFormat, "FORMAT missing at least one ')'");
+      }
       return '\n';
     }
     return format_[offset_++];