fix warnings: expression result unused [-Wunused-value]

In the assert()s take advantage of the fact that string constants
("string") are effectively of type 'const char []', which when used in
an expression yield a non-NULL pointer.

An assertion that should always fail can thus be formulated as:
  assert(!"fail);

An assertion where a text message should be added to the expression can
be written as such:
  assert("message" && expression);

Signed-off-by: Giel van Schijndel <me@mortis.eu>

src/db.h

@@ -88,7 +88,7 @@ protected:
         if (!pdb)
             return false;
         if (fReadOnly)
-            assert(("Write called on database in read-only mode", false));
+            assert(!"Write called on database in read-only mode");
 
         // Key
         CDataStream ssKey(SER_DISK);
@@ -117,7 +117,7 @@ protected:
         if (!pdb)
             return false;
         if (fReadOnly)
-            assert(("Erase called on database in read-only mode", false));
+            assert(!"Erase called on database in read-only mode");
 
         // Key
         CDataStream ssKey(SER_DISK);