utf8_to_uv_msgs: Error if tries to both return and warn

This panics if the flags to the call of this function are contradictory,
in that it is both to warn if anything goes wrong, and not to warn under
any circumstances but instead to return to the caller objects describing
what it would warn.

I'm unsure what the proper thing is to do here.  This commit makes this
a run time panic when the conflict actually happens, but not until.
This is along the lines of you can divide, but only when the divisor is
0 is an error raised.  I think this is better than croaking if there is
even the possibility of a problem.  But I'm not sure.  And should it be
a panic or just a fatal error, or just a warning and we choose probably
to die if one of the flags says to die, or to both warn and return if
one of the flags says just warn.

Author: khwilliamson

utf8.c

@@ -1541,9 +1541,9 @@ to a variable which has been declared to be an C<AV*>, and into which the
 function creates a new AV to store information, described below, about all
 the malformations that were encountered.
 
-If the flag C<UTF8_CHECK_ONLY> is passed, this parameter is ignored.
-Otherwise, when this parameter is set, the flags C<UTF8_DIE_IF_MALFORMED> and
-C<UTF8_FORCE_WARN_IF_MALFORMED> are ignored.
+When this parameter is non-NULL, the C<UTF8_DIE_IF_MALFORMED> and
+C<UTF8_FORCE_WARN_IF_MALFORMED> flags are forbidden, and generate a runtime
+error.  The C<UTF8_CHECK_ONLY> flag is ignored.
 
 What is considered a malformation is affected by C<flags>, the same as
 described in C<L</utf8_to_uv_flags>>.  No array element is generated for
@@ -1640,8 +1640,8 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
     if (UNLIKELY(msgs)) {
         *msgs = NULL;
 
-        /* The msgs parameter has higher priority than these flags */
-        flags &= ~(UTF8_DIE_IF_MALFORMED|UTF8_FORCE_WARN_IF_MALFORMED);
+        /* This form of the function has higher priority than this flag */
+        flags &= ~UTF8_CHECK_ONLY;
     }
 
     /* Each of the affected Hanguls starts with \xED */
@@ -2493,6 +2493,15 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
              * returned; each case statement in the switch above does a
              * continue if no message for it need be generated. */
             if (msgs) {
+                if (flags &
+                        (UTF8_DIE_IF_MALFORMED|UTF8_FORCE_WARN_IF_MALFORMED))
+                {
+                    Perl_croak_nocontext(
+                        "panic: utf8_to_uv_msgs() does not accept either of"
+                        " the UTF8_DIE_IF_MALFORMED or"
+                        " UTF8_FORCE_WARN_IF_MALFORMED flags");
+                }
+
                 if (msgs_return == NULL) {
                     msgs_return = newAV();
                 }