Fix failing test builds with -Werror=maybe-uninitialized on gcc

Apparently the added constexpr makes gcc's control flow analysis to go deeper.

If from_chars returns with error then the out parameter remains
uninitialized. As it's undefined behavior to use its value, it's better
to just skip the rest of the loop in this case.

tests/long_random64.cpp

@@ -62,6 +62,7 @@ void random_values(size_t N) {
         if (errors > 10) {
           abort();
         }
+        continue;
       }
       if (std::isnan(v)) {
         if (!std::isnan(result_value)) {

tests/random64.cpp

@@ -65,6 +65,7 @@ void random_values(size_t N) {
         if (errors > 10) {
           abort();
         }
+        continue;
       }
       if (std::isnan(v)) {
         if (!std::isnan(result_value)) {