basic/strv: avoid potential UB with references to array[-1]

"""
Given an array a[N] of N elements of type T:
- Forming a pointer &a[i] (or a + i) with 0 ≤ i ≤ N is safe.
- Forming a pointer &a[i] with i < 0 or i > N causes undefined behavior.
- Dereferencing a pointer &a[i] with 0 ≤ i < N is safe.
- Dereferencing a pointer &a[i] with i < 0 or i ≥ N causes undefined behavior.
"""

As pointed by by @medhefgo, here we were forming a pointer to a[-1]. a itself
wasn't NULL, so a > 0, and a-1 was also >= 0, and this didn't seem to cause any
problems. But it's better to be formally correct, especially if we move the
code to src/fundamental/ later on and compile it differently.

Compilation shows no size change (with -O0 -g) on build/systemd, so this should
have no effect whatsoever.

src/test/test-strv.c

@@ -639,8 +639,13 @@ TEST(strv_foreach_backwards) {
         STRV_FOREACH_BACKWARDS(check, (char**) NULL)
                 assert_not_reached();
 
-        STRV_FOREACH_BACKWARDS(check, (char**) { NULL })
+        STRV_FOREACH_BACKWARDS(check, STRV_MAKE_EMPTY)
                 assert_not_reached();
+
+        unsigned count = 0;
+        STRV_FOREACH_BACKWARDS(check, STRV_MAKE("ONE"))
+                count++;
+        assert_se(count == 1);
 }
 
 TEST(strv_foreach_pair) {