Fix reading byte from char params[-1] position

Fix reading byte from char params[-1] position

When the ->params string is empty - memory access is made on the byte
before allocated buffer (catched by valgrind) - in the case it would
constain 0x20 - it would even overwrite this buffer.
So fix by checking len > 0 before doing such access.
Also slightly optimise this loop from repeated strlen call.