Fix NULL pointer indirection in FilePath

If the FileBase::lookup operation in the constructor of FilePath returns
NULL, subsequent operations (such as isFile()/isFileSystem()) will call
methods on a NULL 'fb' pointer. This commit fixes this issue by adding
explicit NULL checks and a new method in FilePath (exists()).

libraries/mbed/api/FilePath.h

@@ -34,6 +34,7 @@ public:
 
     bool    isFile(void);
     FileLike* file(void);
+    bool    exists(void);
 
 private:
     const char* file_name;

libraries/mbed/common/FilePath.cpp

@@ -36,9 +36,6 @@ FilePath::FilePath(const char* file_path) : file_name(NULL), fb(NULL) {
         file_name++;
     }
 
-    FileBase::lookup(file_system, len);
-
-
     fb = FileBase::lookup(file_system, len);
 }
 
@@ -47,6 +44,8 @@ const char* FilePath::fileName(void) {
 }
 
 bool FilePath::isFileSystem(void) {
+    if (NULL == fb)
+        return false;
     return (fb->getPathType() == FileSystemPathType);
 }
 
@@ -58,6 +57,8 @@ FileSystemLike* FilePath::fileSystem(void) {
 }
 
 bool FilePath::isFile(void) {
+    if (NULL == fb)
+        return false;
     return (fb->getPathType() == FilePathType);
 }
 
@@ -68,4 +69,8 @@ FileLike* FilePath::file(void) {
     return NULL;
 }
 
+bool FilePath::exists(void) {
+    return fb != NULL;
+}
+
 } // namespace mbed

libraries/mbed/common/retarget.cpp

@@ -163,7 +163,9 @@ extern "C" FILEHANDLE PREFIX(_open)(const char* name, int openmode) {
     } else {
         FilePath path(name);
 
-        if (path.isFile()) {
+        if (!path.exists())
+            return -1;
+        else if (path.isFile()) {
             res = path.file();
         } else {
             FileSystemLike *fs = path.fileSystem();