strbuf: set the proper character when creating new nodes

Commit 82501b3 added an early break when a terminal node is found to
incorrect place -- before setting c. This caused trie to be built that
does not correspond to what it points to in buffer, causing incorrect
deduplications:

  # cat /etc/udev/rules.d/99-bug.rules
  ENV{FOO}=="0"
  ENV{xx0}=="BAR"
  ENV{BAZ}=="00"

  # udevadm test
  * RULE /etc/udev/rules.d/99-bug.rules:1, token: 0, count: 2, label: ''
  M ENV match 'FOO' '0'(plain)
  * RULE /etc/udev/rules.d/99-bug.rules:2, token: 2, count: 2, label: ''
  M ENV match 'xx0' 'BAR'(plain)
  * RULE /etc/udev/rules.d/99-bug.rules:3, token: 4, count: 2, label: ''
  M ENV match 'BAZ' 'x0'(plain)
  * END

The addition of "xx0" following "0" will cause a trie like this to be
created:

  c=\0
    c=0 "0"
      c=0 "xx0"  <-- note the c is incorrect here, causing "00" to be
    c=O "FOO"        deduplicated to it
    c=R "BAR"

This in effect caused the usb_modeswitch rule for Huawei modems to never
match and this never be switched to serial mode from mass storage.

src/basic/strbuf.c

@@ -156,12 +156,13 @@ ssize_t strbuf_add_string(struct strbuf *str, const char *s, size_t len) {
                         return off;
                 }
 
+                c = s[len - 1 - depth];
+
                 /* bsearch is not allowed on a NULL sequence */
                 if (node->children_count == 0)
                         break;
 
                 /* lookup child node */
-                c = s[len - 1 - depth];
                 search.c = c;
                 child = bsearch(&search, node->children, node->children_count,
                                 sizeof(struct strbuf_child_entry),