Pacify GCC 9.1

* src/intervals.c (set_intervals_multibyte_1): Omit unused temps.
* src/xdisp.c (display_mode_element): Use !NILP instead of
STRINGP.  This convinces GCC we’re not dereferencing a
possibly-null pointer, and should be a bit faster anyway.

Author: eggert

src/intervals.c

@@ -2334,7 +2334,6 @@ set_intervals_multibyte_1 (INTERVAL i, bool multi_flag,
 
       if (multi_flag)
 	{
-	  ptrdiff_t temp;
 	  left_end_byte
             = advance_to_char_boundary (start_byte + LEFT_TOTAL_LENGTH (i));
 	  left_end = BYTE_TO_CHAR (left_end_byte);
@@ -2355,8 +2354,6 @@ set_intervals_multibyte_1 (INTERVAL i, bool multi_flag,
 
       if (multi_flag)
 	{
-	  ptrdiff_t temp;
-
 	  right_start_byte
             = advance_to_char_boundary (end_byte - RIGHT_TOTAL_LENGTH (i));
 	  right_start = BYTE_TO_CHAR (right_start_byte);

src/xdisp.c

@@ -24026,7 +24026,7 @@ display_mode_element (struct it *it, int depth, int field_width, int precision,
 			       ? string_byte_to_char (elt, bytepos)
 			       : bytepos);
 		    spec = decode_mode_spec (it->w, c, field, &string);
-		    multibyte = STRINGP (string) && STRING_MULTIBYTE (string);
+		    multibyte = !NILP (string) && STRING_MULTIBYTE (string);
 
 		    switch (mode_line_target)
 		      {
@@ -24702,8 +24702,9 @@ percent99 (ptrdiff_t n, ptrdiff_t d)
 
 /* Return a string for the output of a mode line %-spec for window W,
    generated by character C.  FIELD_WIDTH > 0 means pad the string
-   returned with spaces to that value.  Return a Lisp string in
-   *STRING if the resulting string is taken from that Lisp string.
+   returned with spaces to that value.  Set *STRING to be a Lisp
+   string if the resulting string is taken from that Lisp string;
+   otherwise, set *STRING to Qnil.
 
    Note we operate on the current buffer for most purposes.  */