base64: fix cppcheck warnings

[sreimers]

Fixes:

src/base64/b64.c:188:26: warning: Either the condition '!in' is redundant or there is pointer arithmetic with NULL pointer. [nullPointerArithmeticRedundantC
heck]                        
 const char *in_end = in + ilen;
                         ^ 
src/base64/b64.c:191:6: note: Assuming that condition '!in' is not redundant
 if (!in || !out || !olen)                                                    
     ^  
src/base64/b64.c:188:26: note: Null pointer addition
 const char *in_end = in + ilen;                                              
                         ^