Fix EBCIDIC deficiency in uvoffuni_to_utf8_flags_msgs

By adjusting the case labels, and inserting an extra EBCDIC-only case,
the code can be made to not, e.g. test for surrogates on UTF-EBCDIC
strings that are too short for those.

This is a minor issue, but this is a minor fix.

utf8.c

@@ -274,14 +274,14 @@ Perl_uvoffuni_to_utf8_flags_msgs(pTHX_ U8 *d, UV input_uv, const UV flags, HV**
 
       default:
         p = d + utf8_skip - 1;
-        while (p >= d + 4) {
+        while (p >= d + 4 + ONE_IF_EBCDIC_ZERO_IF_NOT) {
             *p-- = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
             shifted_uv >>= SHIFT;
         }
 
         /* FALLTHROUGH */
 
-      case 4:
+      case 4 + ONE_IF_EBCDIC_ZERO_IF_NOT:
         if (UNLIKELY(UNICODE_IS_SUPER(input_uv))) {
             if (UNLIKELY(      input_uv > MAX_LEGAL_CP
                          && ! (flags & UNICODE_ALLOW_ABOVE_IV_MAX)))
@@ -326,11 +326,12 @@ Perl_uvoffuni_to_utf8_flags_msgs(pTHX_ U8 *d, UV input_uv, const UV flags, HV**
             }
         }
 
-        d[3] = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
+        d[3 + ONE_IF_EBCDIC_ZERO_IF_NOT]
+                                = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
         shifted_uv >>= SHIFT;
         /* FALLTHROUGH */
 
-      case 3:
+      case 3 + ONE_IF_EBCDIC_ZERO_IF_NOT:
         if (input_uv >= UNICODE_SURROGATE_FIRST) {
             if (UNLIKELY(UNICODE_IS_NONCHAR(input_uv))) {
                 HANDLE_UNICODE_NONCHAR(input_uv, flags, msgs);
@@ -340,10 +341,20 @@ Perl_uvoffuni_to_utf8_flags_msgs(pTHX_ U8 *d, UV input_uv, const UV flags, HV**
             }
         }
 
+        d[2 + ONE_IF_EBCDIC_ZERO_IF_NOT]
+                                = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
+        shifted_uv >>= SHIFT;
+        /* FALLTHROUGH */
+
+#ifdef EBCDIC
+
+      case 3:
         d[2] = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
         shifted_uv >>= SHIFT;
         /* FALLTHROUGH */
 
+#endif
+
       case 2:
         d[1] = I8_TO_NATIVE_UTF8((shifted_uv & MASK) | MARK);
         shifted_uv >>= SHIFT;