vector: Prevent NULL argument to memcpy.

Headers declare that memcpy does not accept NULL argument for the first
two parameters.  Add a conditional block to prevent memcpy and ast_free
from running on vectors with NULL element array.

ASTERISK-26526 #close

Change-Id: I988a476bb5fcfcbd3f6d6c6b3e7769e4f9629b71

include/asterisk/vector.h

@@ -172,9 +172,11 @@
 			typeof((vec)->elems) new_elems = ast_calloc(1,		\
 				new_max * sizeof(*new_elems));					\
 			if (new_elems) {									\
+				if ((vec)->elems) {								\
 					memcpy(new_elems, (vec)->elems,				\
 						(vec)->current * sizeof(*new_elems)); 	\
 					ast_free((vec)->elems);						\
+				}												\
 				(vec)->elems = new_elems;						\
 				(vec)->max = new_max;							\
 			} else {											\