🚨 fix a linter warning

Coverity detected two "Memory - illegal accesses  (OVERRUN)" issues. Resizing the buffer should silence this warning.

test/src/unit-to_chars.cpp

@@ -359,7 +359,7 @@ TEST_CASE("formatting")
     {
         auto check_float = [](float number, const std::string & expected)
         {
-            char buf[32];
+            char buf[33];
             char* end = nlohmann::detail::to_chars(buf, buf + 32, number);
             std::string actual(buf, end);
 
@@ -419,7 +419,7 @@ TEST_CASE("formatting")
     {
         auto check_double = [](double number, const std::string & expected)
         {
-            char buf[32];
+            char buf[33];
             char* end = nlohmann::detail::to_chars(buf, buf + 32, number);
             std::string actual(buf, end);