RichString: implement safe rewind

The current rewind logic causes issues when rewinding over the short
string optimization boundary.

ProcessList.c

@@ -113,7 +113,7 @@ void ProcessList_printHeader(const ProcessList* this, RichString* header) {
 
       RichString_appendWide(header, color, alignedProcessFieldTitle(fields[i]));
       if (key == fields[i] && RichString_getCharVal(*header, RichString_size(header) - 1) == ' ') {
-         header->chlen--;  // rewind to override space
+         RichString_rewind(header, 1);  // rewind to override space
          RichString_appendnWide(header,
                                 CRT_colors[PANEL_SELECTION_FOCUS],
                                 CRT_treeStr[Settings_getActiveDirection(this->settings) == 1 ? TREE_STR_DESC : TREE_STR_ASC],

RichString.c

@@ -46,6 +46,10 @@ static void RichString_setLen(RichString* this, int len) {
    }
 }
 
+void RichString_rewind(RichString* this, int count) {
+   RichString_setLen(this, this->chlen - count);
+}
+
 #ifdef HAVE_LIBNCURSESW
 
 static inline int RichString_writeFromWide(RichString* this, int attrs, const char* data_c, int from, int len) {

RichString.h

@@ -42,6 +42,8 @@ typedef struct RichString_ {
    int highlightAttr;
 } RichString;
 
+void RichString_rewind(RichString* this, int count);
+
 void RichString_setAttrn(RichString* this, int attrs, int start, int charcount);
 
 int RichString_findChar(const RichString* this, char c, int start);