guard invalid FB allocs

src/render/Framebuffer.cpp

@@ -3,6 +3,7 @@
 
 bool CFramebuffer::alloc(int w, int h) {
     bool firstAlloc = false;
+    RASSERT((w > 1 && h > 1), "cannot alloc a FB with negative / zero size! (attempted %ix%i)", w, h);
     
     if (m_iFb == (uint32_t)-1) {
         firstAlloc = true;