top: provide for zero length character in utf8 support

This commit is prompted by the preceding change to the
library's escape.c module which, in turn, was prompted
by that issue shown below (with thanks to Konstantin).

Reference(s):
https://gitlab.com/procps-ng/procps/-/issues/176

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/top.c

@@ -710,7 +710,8 @@ static inline int utf8_cols (const unsigned char *p, int n) {
 
    if (n > 1) {
       (void)mbtowc(&wc, (const char *)p, n);
-      if ((n = wcwidth(wc)) < 1) n = 1;
+      // allow a zero as valid, as with a 'combining acute accent'
+      if ((n = wcwidth(wc)) < 0) n = 1;
    }
    return n;
 #else