Fix missing initialization

Found by compiling with gcc 4.8, which shows a warning for use in the
inlined function called in that function.

Closes #155

libobs/callback/calldata.c

@@ -181,7 +181,7 @@ bool calldata_getdata(calldata_t data, const char *name, void *out, size_t size)
 void calldata_setdata(calldata_t data, const char *name, const void *in,
 		size_t size)
 {
-	uint8_t *pos;
+	uint8_t *pos = NULL;
 
 	if (!data || !name || !*name)
 		return;