Address Robert's feedback

@@ -99,10 +99,14 @@
 static boolean_t
 str_reserve(str_t *s, size_t amt)
 {
         size_t newlen = s->str_len + amt;
 
+        /* overflow check */
+        if (newlen < s->str_len || newlen < amt)
+                return (B_FALSE);
+
         if ((amt > 0) && (s->str_len + amt <= s->str_size))
                 return (B_TRUE);
 
         size_t newsize = roundup(newlen, STR_CHUNK_SZ);
         void *temp;

@@ -161,11 +165,11 @@
         dest->str_len += src->str_len;
         return (B_TRUE);
 }
 
 boolean_t
-str_append_c(str_t *s, int c)
+str_append_c(str_t *s, char c)
 {
         if (!str_reserve(s, 1))
                 return (B_FALSE);
 
         s->str_s[s->str_len++] = c;

@@ -207,13 +211,23 @@
         }
 
         if (!str_reserve(dest, src->str_len))
                 return (B_FALSE);
 
-        /* Unlike some programmers, *I* can read manpages. */
+        /*
+         * Shift the contents of dest over at the insertion point.  Since
+         * src and dest ranges will overlap, and unlike some programmers,
+         * *I* can read man pages - memmove() is the appropriate function
+         * to this.
+         */
         (void) memmove(dest->str_s + idx + src->str_len, dest->str_s + idx,
             dest->str_len - idx);
+
+        /*
+         * However the content to insert does not overlap with the destination
+         * so memcpy() is fine here.
+         */
         (void) memcpy(dest->str_s + idx, src->str_s, src->str_len);
         dest->str_len += src->str_len;
 
         return (B_TRUE);
 }