Check that a filename was specified also when using force read

Corresponding to flashrom svn r322 and coreboot v2 svn r3647.

Signed-off-by: Peter Stuge <peter@stuge.se>
Acked-by: Peter Stuge <peter@stuge.se>
diff --git a/flashrom.c b/flashrom.c
index a6d6e91..093f2ea 100644
--- a/flashrom.c
+++ b/flashrom.c
@@ -449,6 +449,10 @@
 				printf("Run flashrom -L to view the hardware supported in this flashrom version.\n");
 				exit(1);
 			}
+			if (!filename) {
+				printf("Error: No filename specified.\n");
+				exit(1);
+			}
 			size = flashes[0]->total_size * 1024;
 			buf = (uint8_t *) calloc(size, sizeof(char));