Fix for GitHub issue #79 - literal numbers must have at least one digit.
Fix for GitHub issue #79 - literal numbers must have at least one digit.

The bug report was for an assertion failure when a number contained only
a lowline (e.g. 'b_), but the standard says that a number can't start
with a lowline (e.g. 'b_1). The parser already rejected these cases for
decimal numbers, but allowed them through for binary/octal/hex numbers.

themperek/iverilog,CastMi/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog