[guid.cpp] string->guid should disallow 'g' char

off-by-1 error in alpha test. only 'a'-'f' should be allowed.

libgnucash/engine/guid.cpp

@@ -92,7 +92,7 @@ char_to_num (unsigned char c) noexcept
 {
     unsigned int digit = c - '0';
     unsigned int alpha = (c | 0x20) - 'a';
-    return digit <= 9 ? digit : alpha <= 6 ? alpha + 10 : -1;
+    return digit <= 9 ? digit : alpha <= 5 ? alpha + 10 : -1;
 }
 
 static inline bool

libgnucash/engine/test/test-gnc-guid.cpp

@@ -74,6 +74,13 @@ TEST (GncGUID, from_string)
     guid = gnc::GUID::create_random ();
     std::string bogus {"Have a great big roast beef sandwich, if you please!"};
     EXPECT_THROW (gnc::GUID::from_string (bogus), gnc::guid_syntax_exception);
+
+    std::string test1 {"0123456789abcdef0123456789abcdef"};
+    auto guid1 = gnc::GUID::from_string (test1);
+    EXPECT_EQ (test1, guid1.to_string());
+
+    std::string test2 {"0123456789abcdef0123456789abcdeg"};
+    EXPECT_THROW (gnc::GUID::from_string (test2), gnc::guid_syntax_exception);
 }
 
 TEST (GncGUID, round_trip)