BUILD: pattern: silence a warning about uninitialized value

Gcc 3.4 warns that mask may be used uninitialized in pattern.c. This
is wrong since it's used in the same condition as its assignment,
although it's not necessarily obvious for the compiler. Fix this by
initializing the value.

This was introduced by recent commit 01cdcd4a so no backport is needed.
diff --git a/src/pattern.c b/src/pattern.c
index d667cef..ce60f76 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -1117,7 +1117,7 @@
 	struct pat_idx_elt *elt;
 	const char *args[2];
 	int opaque = 0;
-	unsigned int mask;
+	unsigned int mask = 0;
 
 	/* no real pattern */
 	if (!expr->match || expr->match == pat_match_nothing)