Silence g++-7.4 warning (Ubuntu 18)

Which is a wrong warning, which came with LTO and -O3.
Newer g++ can detect that it is initialized in this branch

Author: rurban

src/util/format_number_range.cpp

@@ -99,6 +99,10 @@ static void append_numbers_and_reset(
         "lower bound must not be larger than upper bound '" +
         std::to_string(*begin_range) + "-" + std::to_string(*number) + "'");
     }
+    // old g++ with -O3 falsely complain for *begin_range (g++-7.4)
+#if __GNUC__ > 4
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
     for(unsigned i = *begin_range; i < *number; ++i)
       numbers.push_back(i);
     // add upper bound separately to avoid