Consider strings validation for getUniformIndices and getUniformBlockIndex

[lexaknyazev]

Another follow-up to #3727.

The current WebGL 2.0 specs do not apply the same string validation rules (max length and allowed characters) to the elements of the uniformNames array parameter of the getUniformIndices command and to the uniformBlockName parameter of the getUniformBlockIndex command as to the name parameter of getAttribLocation or getUniformLocation. This is likely an oversight.