Fix in /R command

Hello,
there is a segfault in /R command caused by a dereferencing of NULL pointer.

This patch fix it.

I did also changes in comparison function for ROP grep matching. 
strncasecmp test only the first characters, but for ROP searching, sometimes we want to look for a specific register.

For example, the gadget `mov ecx, [rax]` was not matched by /R rax.

To do this, I implemented some string functions in the radare API : r_strncasestr, r_strnstr, r_strcasecmp which can be used in other way.

I hope this patch will be useful for radare project and radare users :) 

-TOSH-