Fix out-of-bounds access (#19)

[sebastiansturm]

at least one language server (ccls) uses symbol kinds greater than 26 that, using the default value of lsp-ivy-symbol-kind-to-face, will trigger out-of-bounds errors (see issue #19). This PR avoids the issue by remapping anything beyond (length lsp-ivy-symbol-kind-to-face) to entry 0 (unknown - empty label).

Personally, I've never seen this issue so perhaps it's rare and the few (?) users who have run into it have for the most part decided to just resize their lsp-ivy-symbol-kind-to-face accordingly and fill in the interesting slots. Or perhaps this is a common issue and lsp-ivy-symbol-kind-to-face should be changed to an associative container to more conveniently extend to discontinuous value ranges as used by ccls. Opinions welcome