No error handling of binary/hexadecimal floating literals

[lifthrasiir]

Due to the lack of checking (i.e. right before libsyntax/parse/lexer.rs:434), the current lexer parses 0x12.3 as a synonym to 12.3 (!). Moreover 0xAA.5 triggers an assertion failure from APFloat. Tangentially related to issue #1433.

[catamorphism]

I'm going to assume that d9a61f2 fixes this, and close. Reopen if I'm wrong!