core/load-fragment: avoid allocating 0 bytes when given an invalid command

With a command line like "@/something" we would allocate an array with
0 elements. Avoid that, and add a test too.

(cherry picked from commit e01ff42899)

Conflicts:
	src/core/load-fragment.c
	src/test/test-unit-file.c

src/core/load-fragment.c

@@ -581,7 +581,8 @@ int config_parse_exec(const char *unit,
                 }
 
         found:
-                n = new(char*, k + !honour_argv0);
+                /* If seperate_argv0, we'll move first element to path variable */
+                n = new(char*, MAX(k + !honour_argv0, 1u));
                 if (!n)
                         return log_oom();