valgrind vs genl_putmixed

valgrind vs genl_putmixed

In the midst of composing a commit message about how I reorganized some
of genl_putmixed()'s code without finding any problem, I realized that
there was a problem.  The character immediately after \G12345678 would
be copied directly to the output buffer without examination.  If that
was the leading backslash for a second encoded sequence, the G and the
hex digits would follow their backslash as just ordinary chars, which
is not what's intended.  Or if instead of a backslash the next character
was the input's terminating '\0', the latter would be copied into the
output and the pointer to the input string would be incremented, then
the next loop iteraction would examined whatever followed.  If valgrind
is smart enough--and it seems to be--it would complain about accessing
a character that putmixed()'s caller hadn't initialized.

The only use of putmixed() I'm sure about is the what-is code showing
a screen symbol with its explanation, which doesn't exercise either
\G12345678\G12345678 or \G12345678\0.  I didn't go hunting to see if
there was someplace that might have an encoded symbol at the end of the
string.  what-is still works after this patch....

The only substantive change is adding ``continue'' but I haven't gone
back and undone the reorg that preceded it.