bsps/riscv/shared/fdt/riscv-fdt.c: Fix may be uniitialized warning

The variables start and size were used in the return statement
even if none of the if's set them. Defaulted them to zero which
results in returning NULL on error which was expected anyway.

bsps/riscv/shared/fdt/riscv-fdt.c

@@ -49,8 +49,8 @@ void *riscv_fdt_get_end_of_memory( const void *fdt )
   int ac;
   int sc;
   int len;
-  uintptr_t start;
-  uintptr_t size;
+  uintptr_t start = 0;
+  uintptr_t size = 0;
 
   node = fdt_path_offset( fdt, "/memory" );