[FuzzMutate] Bailout from injecting into empty basic blocks.

In rare cases we can receive request to inject into completelly empty basic block. In the normal case 
all basic blocks contain at least terminator instruction, but it is possible that the only instruction is 
catchpad instruction which is not part of the instruction iterator. This case seems rare enough to not care
about it.
Submiting without review, since it seems almost NFC. I couldn't come up with any reasonable way to test this.

llvm-svn: 319444