Fix -Wno-sometimes-uninitialized by initializing "result" in parse_sax

The function parse_sax() returns sometimes-uninitialized warnings in
some compilers. This commit fixes that.

include/nlohmann/detail/input/binary_reader.hpp

@@ -64,7 +64,7 @@ class binary_reader
                    const bool strict = true)
     {
         sax = sax_;
-        bool result;
+        bool result = false;
 
         switch (format)
         {

single_include/nlohmann/json.hpp

@@ -6033,7 +6033,7 @@ class binary_reader
                    const bool strict = true)
     {
         sax = sax_;
-        bool result;
+        bool result = false;
 
         switch (format)
         {