libmodprobe: Fail when modules.dep lacks colon

The first argument in a modules.dep line must end with a colon so fail
if that condition is not met.

Test: libmodprobe_tests
Change-Id: I6f3a22758302f16b924e5a16f7af9bf35f1a56f3
diff --git a/libmodprobe/libmodprobe.cpp b/libmodprobe/libmodprobe.cpp
index ceabf62..b3ae937 100644
--- a/libmodprobe/libmodprobe.cpp
+++ b/libmodprobe/libmodprobe.cpp
@@ -66,6 +66,7 @@
         deps.emplace_back(prefix + args[0].substr(0, pos));
     } else {
         LOG(ERROR) << "dependency lines must start with name followed by ':'";
+        return false;
     }
 
     // Remaining items are dependencies of our module