quotearg: avoid uninitialized variable use

quotearg: avoid uninitialized variable use

Coverity correctly deduced:

Error: UNINIT:
m4-1.4.16/lib/quotearg.c:171: var_decl: Declaring variable "o" without initializer.
m4-1.4.16/lib/quotearg.c:175: uninit_use: Using uninitialized value "o": field "o".right_quote is uninitialized.

When custom_quoting_style was introduced in commit 12247f77,
this method was not updated, and any caller that passed
the new enum value to any of the existing quotearg_*style
functions could trigger a crash from the uninitialized memory.
That was already documented as unspecified behavior, though,
so changing to an abort makes it easier to spot bad code that
passes the wrong enum value, rather than waiting for the
eventual bad memory dereference later on.

Most callers of quotearg_*style were using quoting_style_args
and quoting_style_vals to map strings to particular enum
values, and custom_quoting_style is (intentionally) not covered
by these arrays, so the pre-patch bug/post-patch abort are not
possible with those callers.

* lib/quotearg.c (quoting_options_from_style): Initialize
remaining fields, and ensure that custom styles are only used via
quoting_options rather than quoting_style.

Signed-off-by: Eric Blake <eblake@redhat.com>