search: trim a failed search string for width instead of length

When reducing the search string to at most half the screen width,
it should employ its width in columns, not its length in bytes.

Also adjust the type of a variable.

This fixes http://savannah.gnu.org/bugs/?52057.

src/search.c

@@ -70,12 +70,12 @@ void regexp_cleanup(void)
 void not_found_msg(const char *str)
 {
     char *disp;
-    int numchars;
+    size_t numchars;
 
     assert(str != NULL);
 
     disp = display_string(str, 0, (COLS / 2) + 1, FALSE);
-    numchars = actual_x(disp, mbstrnlen(disp, COLS / 2));
+    numchars = actual_x(disp, strnlenpt(disp, COLS / 2));
 
     statusline(HUSH, _("\"%.*s%s\" not found"), numchars, disp,
 		(disp[numchars] == '\0') ? "" : "...");