bit: avoid calling strdup(NULL)

It seems the behavior of strdup is undefined when NULL is passed in.
Clang's static analyzer complains about this:

> frameworks/base/tools/bit/command.cpp:195:25: warning: Null pointer
passed to 1st parameter expecting 'nonnull'
[clang-analyzer-core.NonNullParamChecker]

Since we don't seem to care about malloc failures (which is a good thing
IMO), adding a second check for the successful completion of `strdup`
seems pointless.

Bug: 206470603
Test: TreeHugger
Change-Id: Ib621659e6fb600203694974f02d96ba0acb3362f

tools/bit/command.cpp

@@ -192,10 +192,11 @@ exec_with_path_search(const char* prog, char const* const* argv, char const* con
     if (strchr(prog, '/') != NULL) {
         return execve(prog, (char*const*)argv, (char*const*)envp);
     } else {
-        char* pathEnv = strdup(getenv("PATH"));
-        if (pathEnv == NULL) {
+        const char* pathEnvRaw = getenv("PATH");
+        if (pathEnvRaw == NULL) {
             return 1;
         }
+        char* pathEnv = strdup(pathEnvRaw);
         char* dir = pathEnv;
         while (dir) {
             char* next = strchr(dir, ':');