retarget: Fix path behaviour without leading slash

Current behaviour ends up undefined when the constructor leaves early.
Now FilePath just discards leading slashes and otherwise asserts if the
path is bad.

Author: geky

platform/FilePath.cpp

@@ -18,9 +18,10 @@
 namespace mbed {
 
 FilePath::FilePath(const char* file_path) : file_name(NULL), fb(NULL) {
-    if ((file_path[0] != '/') || (file_path[1] == 0)) return;
+    // skip slashes
+    file_path += strspn(file_path, "/");
 
-    const char* file_system = &file_path[1];
+    const char* file_system = file_path;
     file_name = file_system;
     int len = 0;
     while (true) {
@@ -36,6 +37,7 @@ FilePath::FilePath(const char* file_path) : file_name(NULL), fb(NULL) {
         file_name++;
     }
 
+    MBED_ASSERT(len != 0);
     fb = FileBase::lookup(file_system, len);
 }