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/main.cpp

@@ -1688,7 +1688,7 @@ string GetWarnings(string strFor)
         return strStatusBar;
     else if (strFor == "rpc")
         return strRPC;
-    assert(("GetWarnings() : invalid parameter", false));
+    assert(!"GetWarnings() : invalid parameter");
     return "error";
 }