Fix build in GCC 7

The following error is emitted by gcc 7.3:

include/internal/catch_tostring.cpp:217:21: error: comparison is always true due to limited range of data type [-Werror=type-limits]
|      } else if ('\0' <= value && value < ' ') {
|                 ~~~~~^~~~~~~~
| cc1plus: all warnings being treated as errors

We can drop the first part of the if since '\0' == 0 and it will always
be <= than value of char (which is unsigned by default).

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

include/internal/catch_tostring.cpp

@@ -214,7 +214,7 @@ std::string StringMaker<char>::convert(char value) {
         return "'\\n'";
     } else if (value == '\t') {
         return "'\\t'";
-    } else if ('\0' <= value && value < ' ') {
+    } else if (value < ' ') {
         return ::Catch::Detail::stringify(static_cast<unsigned int>(value));
     } else {
         char chstr[] = "' '";