Fix a latent bug in print_wchar

Fix a latent bug in print_wchar

print_wchar keeps track of when escape sequences are emitted, to force
an escape sequence if needed by a subsequent character.  For example
for the string concatenation "\0" "1", gdb will print "\000\061" --
because printing "\0001" might be confusing.

However, this code has two errors.  First, this logic is not needed
for octal escapes, because there is a length limit of 3 for octal
escapes, and gdb always prints these with "%.3o".  Second, though,
this *is* needed for hex escapes, because those do not have a length
limit.

This patch fixes these problems and adds the appropriate tests.