libstdc++: Avoid -Wmaybe-uninitialized warnings in text_encoding.cc

libstdc++: Avoid -Wmaybe-uninitialized warnings in text_encoding.cc

These variables are only read from if we haven't reached the end of
either range, in which case they're guaranteed to be initialized to the
next alphanumeric character. But we can just initialize them to make the
compiler happy.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (__charset_alias_match): Initialize
	__var_a and __var_b.