style: align stats/base/dists/negative-binomial/mean with namespace conventions

Merged the two-block argument validation prologue in `lib/main.js` and
`src/main.c` into a single `if`-block, matching the convention used by
10/12 sibling packages in `@stdlib/stats/base/dists/negative-binomial`
(83% conformance) and 6/7 native-binding C siblings (86% conformance).
Behavior is unchanged: invalid inputs still return NaN.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/negative-binomial/mean/lib/main.js

@@ -63,11 +63,7 @@ var isnan = require( '@stdlib/math/base/assert/is-nan' );
 function mean( r, p ) {
 	if (
 		isnan( r ) ||
-		isnan( p )
-	) {
-		return NaN;
-	}
-	if (
+		isnan( p ) ||
 		r <= 0.0 ||
 		p < 0.0 ||
 		p > 1.0

lib/node_modules/@stdlib/stats/base/dists/negative-binomial/mean/src/main.c

@@ -33,11 +33,7 @@
 double stdlib_base_dists_negative_binomial_mean( const double r, const double p ) {
 	if (
 		stdlib_base_is_nan( r ) ||
-		stdlib_base_is_nan( p )
-	) {
-		return 0.0 / 0.0; // NaN
-	}
-	if (
+		stdlib_base_is_nan( p ) ||
 		r <= 0.0 ||
 		p < 0.0 ||
 		p > 1.0