Fix return value of operator= in BigUnsignedInABase.

PiperOrigin-RevId: 255794761
Change-Id: I915a361d787bad038a2ddb9955aeaae75c2a929e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/56990
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/bigint/BigUnsignedInABase.hh b/third_party/bigint/BigUnsignedInABase.hh
index dbd2cf9..fb293ba 100644
--- a/third_party/bigint/BigUnsignedInABase.hh
+++ b/third_party/bigint/BigUnsignedInABase.hh
@@ -64,9 +64,10 @@
 	BigUnsignedInABase(const BigUnsignedInABase &x) : NumberlikeArray<Digit>(x), base(x.base) {}
 
 	// Assignment operator
-	void operator =(const BigUnsignedInABase &x) {
+	BigUnsignedInABase& operator =(const BigUnsignedInABase &x) {
 		NumberlikeArray<Digit>::operator =(x);
 		base = x.base;
+		return *this;
 	}
 
 	// Constructor that copies from a given array of digits.