installd: Avoid potential use of null 'src'.

In create_cache_path, we have code checking whether 'src' is null,
implying it's possibile it could be null here.  To be safe, we move
our strlen call until after this check.

Change-Id: I6a67cdad8ebf8bdce1562ce5f0cbf98b084807ef
diff --git a/cmds/installd/installd.cpp b/cmds/installd/installd.cpp
index 7ca7527..38290cc 100644
--- a/cmds/installd/installd.cpp
+++ b/cmds/installd/installd.cpp
@@ -128,13 +128,13 @@
 bool create_cache_path(char path[PKG_PATH_MAX],
                        const char *src,
                        const char *instruction_set) {
-    size_t srclen = strlen(src);
-
-        /* demand that we are an absolute path */
-    if ((src == 0) || (src[0] != '/') || strstr(src,"..")) {
+    /* demand that we are an absolute path */
+    if ((src == nullptr) || (src[0] != '/') || strstr(src,"..")) {
         return false;
     }
 
+    size_t srclen = strlen(src);
+
     if (srclen > PKG_PATH_MAX) {        // XXX: PKG_NAME_MAX?
         return false;
     }