GetDirectoryPath: return NULL, don't crash when no slash

Noted in #634.

src/core.c

@@ -1374,6 +1374,9 @@ const char *GetDirectoryPath(const char *fileName)
     memset(filePath, 0, 256);
 
     lastSlash = strprbrk(fileName, "\\/");
+    if (!lastSlash)
+        return NULL;
+
     strncpy(filePath, fileName, strlen(fileName) - (strlen(lastSlash) - 1));
     filePath[strlen(fileName) - strlen(lastSlash)] = '\0';