Improve the detection of missing required fields.

Now the limit of tracked fields is configurable at compile-time using
PB_MAX_REQUIRED_FIELDS. Added related test and updated documentation.

Fixes issue #18.

tests/test_missing_fields.c

@@ -0,0 +1,49 @@
+/* Checks that missing required fields are detected properly */
+
+#include <stdio.h>
+#include <pb_encode.h>
+#include <pb_decode.h>
+#include "missing_fields.pb.h"
+
+int main()
+{
+    uint8_t buffer[512] = {};
+    
+    /* Create a message with one missing field */
+    {
+        MissingField msg = {};
+        pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
+        if (!pb_encode(&stream, MissingField_fields, &msg))
+        {
+            printf("Encode failed.\n");
+            return 1;
+        }
+    }
+
+    /* Test that it decodes properly if we don't require that field */
+    {
+        MissingField msg = {};
+        pb_istream_t stream = pb_istream_from_buffer(buffer, sizeof(buffer));
+        
+        if (!pb_decode(&stream, MissingField_fields, &msg))
+        {
+            printf("Decode failed.\n");
+            return 2;
+        }
+    }
+    
+    /* Test that it does *not* decode properly if we require the field */
+    {
+        AllFields msg = {};
+        pb_istream_t stream = pb_istream_from_buffer(buffer, sizeof(buffer));
+        
+        if (pb_decode(&stream, AllFields_fields, &msg))
+        {
+            printf("Decode didn't detect missing field.\n");
+            return 3;
+        }
+    }
+    
+    return 0; /* All ok */
+}
+