Actually fix GH #17370

I only added a test, but not the change in
9f16475.  The test passes except when
run under address sanitizer or valgrind.

(cherry picked from commit dca9f61)

numeric.c

@@ -765,6 +765,9 @@ Perl_grok_infnan(pTHX_ const char** sp, const char* send)
             s++;
 
             flags |= IS_NUMBER_NAN | IS_NUMBER_NOT_INT;
+            if (s == send) {
+                return flags;
+            }
 
             /* NaN can be followed by various stuff (NaNQ, NaNS), but
              * there are also multiple different NaN values, and some