Fix undefined behavior in `String::operator+=(const String &)`
Fix undefined behavior in `String::operator+=(const String &)`

pkowal1982/godot,Zylann/godot,pkowal1982/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,vkbsb/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,pkowal1982/godot,firefly2442/godot,vkbsb/godot,firefly2442/godot,godotengine/godot,ZuBsPaCe/godot,godotengine/godot,ZuBsPaCe/godot,Zylann/godot,Zylann/godot,Shockblast/godot,BastiaanOlij/godot,Shockblast/godot,sanikoyes/godot,firefly2442/godot,BastiaanOlij/godot,akien-mga/godot,Shockblast/godot,josempans/godot,pkowal1982/godot,Shockblast/godot,Zylann/godot,pkowal1982/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,guilhermefelipecgs/godot,akien-mga/godot,josempans/godot,Shockblast/godot,sanikoyes/godot,Shockblast/godot,vkbsb/godot,firefly2442/godot,josempans/godot,vkbsb/godot,Shockblast/godot,ZuBsPaCe/godot,godotengine/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,akien-mga/godot,akien-mga/godot,godotengine/godot,akien-mga/godot,sanikoyes/godot,BastiaanOlij/godot,pkowal1982/godot,josempans/godot,josempans/godot,sanikoyes/godot,vkbsb/godot,firefly2442/godot,ZuBsPaCe/godot,vkbsb/godot,sanikoyes/godot,godotengine/godot,pkowal1982/godot,firefly2442/godot,ZuBsPaCe/godot,godotengine/godot,sanikoyes/godot,josempans/godot,Zylann/godot,akien-mga/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,pkowal1982/godot,vkbsb/godot,Zylann/godot,firefly2442/godot,vkbsb/godot,godotengine/godot,BastiaanOlij/godot,ZuBsPaCe/godot,sanikoyes/godot,ZuBsPaCe/godot,josempans/godot,Shockblast/godot,firefly2442/godot,akien-mga/godot,akien-mga/godot,Zylann/godot,Zylann/godot,sanikoyes/godot,josempans/godot,godotengine/godot,BastiaanOlij/godot