Fix String(Hex()) usage

- additional add test casye to log.run

Fixes #2265

repos/base/include/util/string.h

@@ -592,7 +592,7 @@ class Genode::String
 
 			void out_string(char const *str, size_t n) override
 			{
-				while (n-- > 0 && _capacity_left())
+				while (n-- > 0 && _capacity_left() && *str)
 					_append(*str++);
 			}
 		};

repos/base/run/log.run

@@ -36,5 +36,6 @@ compare_output_to {
 [init -> test-log] negative hex char:  0xfe
 [init -> test-log] positive hex char:  0x02
 [init -> test-log] multiarg string:    "parent -> child.7"
+[init -> test-log] String(Hex(3)):     0x3
 [init -> test-log] Test done.
 }

repos/base/src/test/log/main.cc

@@ -30,5 +30,8 @@ void Component::construct(Genode::Env &env)
 	typedef String<128> Label;
 	log("multiarg string:    ", Label(Char('"'), "parent -> child.", 7, Char('"')));
 
+	String<32> hex(Hex(3));
+	log("String(Hex(3)):     ", hex);
+
 	log("Test done.");
 }