kernel/util: Fix Null pointer passed as 1st argument to memset()

Pointed out by Clang Static Analyzer.

Change-Id: I8ee4c5d6adac129fc7f3d4117081e0ac26dd2ea0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3759
Reviewed-by: Jérôme Duval <[email protected]>

src/system/kernel/util/Bitmap.cpp

@@ -32,10 +32,10 @@ Bitmap::Bitmap(int bitCount)
 	if (fBits == NULL) {
 		fSize = 0;
 		fInitStatus = B_NO_MEMORY;
+	} else {
+		fElementsCount = count;
+		memset(fBits, 0, sizeof(addr_t) * count);
 	}
-
-	fElementsCount = count;
-	memset(fBits, 0, sizeof(addr_t) * count);
 }