Fix too-short {prefix} evaluation

[1ec5]

Fixed an issue where the {prefix} token in tile URL templates was evaluated incorrectly when requesting a source’s tiles. The prefix ended up being one character long, even though it should be two characters long (compare with GL JS):

mapbox-gl-native/platform/darwin/docs/guides/Tile URL Templates.md.ejs

Lines 100 to 102 in 53bbf9d

	<td><code>{prefix}</code></td>
	<td>Two hexadecimal digits chosen such that each visible tile has a
	different prefix. The prefix is typically used for domain sharding.</td>

This appears to have been an attempt to use the std::string fill constructor, but it ended up creating a one-character-long string consisting of ␂ and attempting to overwrite the null terminator, which resulted in a one-character-long prefix and a tile that couldn’t be found.

/cc @kkaefer @tobrun

[kkaefer]

Ooof, good catch. It looks like this is a pretty bad trap in the std::string constructor. Instead of choosing the string(size_t, char) constructor that I intended, it implicitly converted 2 to a std::initializer_list<char> and chose string(std::initializer_list<char>). This was only possibly because I was using list initialization with braces, which chooses the initializer_list constructor in this case. This wouldn't have happened with std::string prefix(2) (compiler error). Thanks for catching this! I assume our memory sanitizer didn't catch this because we weren't technically writing out of bounds; just overwriting the null terminator in the string.