Avoid assertion violations in STRING_CHAR

* src/xdisp.c (handle_composition_prop):
* src/editfns.c (styled_format): Don't call 'STRING_CHAR' on
unibyte strings.  This avoids assertion violation in
'string_char_and_length'.

(cherry picked from commit d52d6e1e106117eb4bba81a65e256e2e793037b6)

2 files changed, 7 insertions, 2 deletions

diff --git a/src/editfns.c b/src/editfns.c
index 790f66e3a02..203d3a4a57f 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -3468,7 +3468,9 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
                       || conversion == 'o' || conversion == 'x'
                       || conversion == 'X'))
             error ("Invalid format operation %%%c",
-                   STRING_CHAR ((unsigned char *) format - 1));
+                   multibyte_format
+                   ? STRING_CHAR ((unsigned char *) format - 1)
+                   : *((unsigned char *) format - 1));
           else if (! (FIXNUMP (arg) || ((BIGNUMP (arg) || FLOATP (arg))
                                         && conversion != 'c')))
             error ("Format specifier doesn't match argument type");
diff --git a/src/xdisp.c b/src/xdisp.c
index f5f3a811e9d..0d0f77566bf 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -6034,7 +6034,10 @@ handle_composition_prop (struct it *it)
       pos_byte = IT_STRING_BYTEPOS (*it);
       string = it->string;
       s = SDATA (string) + pos_byte;
-      it->c = STRING_CHAR (s);
+      if (STRING_MULTIBYTE (string))
+        it->c = STRING_CHAR (s);
+      else
+        it->c = *s;
     }
   else
     {