pbx_variables: initialize uninitialized variable

The variable cp4 in a variable substitution function
can potentially be used without being initialized
currently. This causes Asterisk to no longer compile.

This initializes cp4 to NULL to make the compiler
happy.

ASTERISK-29803 #close

Change-Id: I392579cbb76db2795d5820c9427cf55fbcee9e72

main/pbx_variables.c

@@ -692,7 +692,7 @@ void pbx_substitute_variables_helper_full_location(struct ast_channel *c, struct
 			int offset;
 			int offset2;
 			int isfunction;
-			char *cp4;
+			char *cp4 = NULL;
 			char workspace[VAR_BUF_SIZE] = "";
 
 			/* We have a variable.  Find the start and end, and determine