fix(printf): fix negative argument precision

Fixes #25

printf.c

@@ -464,7 +464,8 @@ static int _vsnprintf(out_fct_type out, char* buffer, const size_t maxlen, const
         precision = _atoi(&format);
       }
       else if (*format == '*') {
-        precision = (unsigned int)va_arg(va, int);
+        const int prec = (int)va_arg(va, int);
+        precision = prec > 0 ? (unsigned int)prec : 0U;
         format++;
       }
     }

test/test_suite.cpp

@@ -1226,6 +1226,9 @@ TEST_CASE("misc", "[]" ) {
   test::sprintf(buffer, "%.*f", 2, 0.33333333);
   REQUIRE(!strcmp(buffer, "0.33"));
 
+  test::sprintf(buffer, "%.*d", -1, 1);
+  REQUIRE(!strcmp(buffer, "1"));
+
   test::sprintf(buffer, "%.3s", "foobar");
   REQUIRE(!strcmp(buffer, "foo"));