C API: Fixed minor GCC warning
C API: Fixed minor GCC warning

ljx0305/ocilib,vrogier/ocilib,kornefalk/ocilib2017,Convey-Compliance/ocilib,vrogier/ocilib,kornefalk/ocilib2017,alexeyvo/ocilib,kornefalk/ocilib2017,ljx0305/ocilib,alexeyvo/ocilib,Convey-Compliance/ocilib,ljx0305/ocilib,vrogier/ocilib,alexeyvo/ocilib,Convey-Compliance/ocilib,vrogier/ocilib