Do not crash on capitalize with an empty string (#337)

[youdie006]

Fixes #337.

Problem

{{ capitalize("") }} crashes. In Renderer::visit's Op::Capitalize case the code unconditionally writes result[0] and dereferences result.begin() + 1:

auto result = get_arguments<1>(node)[0]->get<json::string_t>();
result[0] = static_cast<char>(::toupper(result[0]));
std::transform(result.begin() + 1, result.end(), result.begin() + 1, ...);

For an empty string result[0] is out of bounds, result.begin() + 1 is an invalid iterator, and the transform range [begin() + 1, end()) is malformed (first > last). This is reachable from ordinary valid template input, so capitalize("") segfaults (a _GLIBCXX_DEBUG build aborts with "function requires a valid iterator range"; ASan reports a stack-buffer-overflow read).

Fix

Guard the capitalization with a non-empty check; an empty argument returns "" (the reporter's expected behavior). Applied to both include/inja/renderer.hpp and the single_include/inja/inja.hpp amalgamation.

Test

Added CHECK(env.render("{{ capitalize(\"\") }}", data) == ""); to the existing capitalize subcase in test/test-functions.cpp. Red-green verified: before the fix the test binary segfaults (exit 139) at this case; after, all 15 test cases / 259 assertions pass, warning-clean under -Wall -Wextra.

[pantor]

Issue was resolved by #343.