msgbuf: Modernize pldm_msgbuf_skip() implementation

The implementation of the other msgbuf APIs follows a pattern that
helps static analysis. Make the implementation of pldm_msgbuf_skip()
consistent, even if it wasn't yet causing dramas.

Change-Id: I5f0486d935935962130a20f3a6564587183fefff
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/msgbuf.h b/src/msgbuf.h
index f953981..b4616b6 100644
--- a/src/msgbuf.h
+++ b/src/msgbuf.h
@@ -1171,16 +1171,17 @@
 	}
 #endif
 
-	if (ctx->remaining < INTMAX_MIN + (intmax_t)count) {
-		return -EOVERFLOW;
+	if (ctx->remaining >= (intmax_t)count) {
+		ctx->cursor += count;
+		ctx->remaining -= (intmax_t)count;
+		return 0;
 	}
-	ctx->remaining -= (intmax_t)count;
-	if (ctx->remaining < 0) {
-		return -EOVERFLOW;
-	}
-	ctx->cursor += count;
 
-	return 0;
+	if (ctx->remaining >= INTMAX_MIN + (intmax_t)count) {
+		ctx->remaining -= (intmax_t)count;
+	}
+
+	return -EOVERFLOW;
 }
 
 /**